Had to use the thumbnail image in the answer box but something like this maybe? (Click link below thumbnail for full sized image)
Source(s):
http://gnuhaus.com/iblog/giraffe_full.jpg

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A disappointed giraffe will always give itself away as it cannot hide it's long face.
Source(s):
The following web page with will cheer up sad giraffes like like giraffe humor:
http://www.funny2.com/bar.htm

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"Where's my dinner, woman?" he said.
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it would have its head hung low with its ears down.
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I think this giraffe looks pretty disapointed and sad.

http://www.corante.com/newyork/archives/Giraffe.jpg
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A disappointed giraffe -long in the face, to match his neck! In need of lengthy therapy.
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http://www.dallasnews.com/sharedcontent/dws/img/v3/01-22-2007.nmc_16Giraffe2.GAE22LI53.1.jpg
Source(s):
http://www.dallasnews.com/sharedcontent/dws/img/v3/01-22-2007.nmc_16Giraffe...

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It would still look like a girafffe :-P Just kidding .... :-)

In my point of view, a dissapointed giraffe would like this

http://www.birminghamzoo.com/images/giraffe.png

or like this

http://www.wildlife-pictures-online.com/image-files/giraffe_tb-6285_blog.jpg

or mostly like this

http://giraffesanctuary.wildlifedirect.org/files/2008/07/baby-giraffe.jpg

Occasionally some express their disappointment like this

http://www.aucklandzoo.co.nz/Homepage/Whats-Happening/News/Winning-name-for-baby-giraffe-announced/images/Giraffe%20Ntombi%20250707%20ZD155349.JPG

* But scientifically, giraffe face is designed that it cannot express any emotions.
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First, you must ask yourself WHY the giraffe is disappointed. Depending on the reason, he could look a number of ways.

Can't reach a leaf: His eyes dart side to side, making sure no one sees that he can't reach the leaf. Then he bumps the tree hoping the leaf will fall. Disappointed when it doesn't he drops his long neck and limps away. He really shouldn't have tried to bump that tree.

Shortest giraffe of all the giraffes: When you see him, he's always stretching. Sometimes he stands on the tips of his hooves, or tries to, so he looks taller. But most of the time he just sits down and orders all the other giraffes around. It's called short giraffe syndome.

Spots are fading: Catching his reflection in a pool of water he notices that his spots just aren't as bright or defined as the younger giraffes. He wonders if it hurts his chances with the ladies. Often he just looks a little unsure, he's slower to assert himself as a giraffe and is likely to go for the lower leaves on the tree.

Didn't get chosen to live in the wildlife park: He did everything right, studied the others that got picked in past years. He was prepared! But somehow he wasn't chosen. You'll often find him in groups, feigning a fake injury and talking about his glory days when he was almost chosen for the park. He's the one who lists his head to the side when there's nothing wrong with him or walks with a slight limp. He's not injured, but his ego is... and he's oh so disappointed.
Source(s):
My imagination and extensive experience with disappointed wildlife

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Disappointed.
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Disappointed Giraffe....
http://www.blisstree.com/playlibrary/files/2009/07/gentle-giraffe_whtbg3.jpg

somewhat disappointed Giraffe....
http://whytheluckystiff.net/i.content/mirador_lion.jpg

very disappointed Giraffe....
http://scienceblogs.com/tetrapodzoology/giraffe%20vs%20plane.jpg
Source(s):
http://images.google.com.co/imgres?imgurl=http://whytheluckystiff.net/i.con...

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First, yes animals have emotions and disappointment is undoubtedly one of them. However, various animals do not all have the same way of expressing each emotion and humans are usually wrong when they try to interpret the emotions of other animals by comparing their facial expressions to human facial expressions.

It is unlikely that giraffes have been studied in such detail that a subtle emotion such as disappointment would have been identified. The major expressions, such as aggression and submission are expressed by the way the giraffe holds its neck.

Giraffes were long thought to be silent but it turns out this is not the case, most of their sounds are too low frequency for humans to hear. It is quite possible that giraffes have a sound for disappointment.

Zoo keepers believe that giraffes convey many emotions through their eyes. This is another likely candidate for the way they express disappointment.

It would be fairly easy to research this at a zoo by photographing giraffes who were disappointed by not receiving an expected treat or similar.

Until that is done, one can only make the educated guess that the giraffe would express disappointment through the expression of its eyes, perhaps accompanied by a vocalization inaudible to us.
Source(s):
http://www.ehow.com/how-does_4567917_giraffes-communicate.html
http://www.animalvoice.com/Giraffe.htm
http://spot.colorado.edu/~humphrey/fact%20sheets/giraffe/giraffe.htm

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There are a lot of great answers so far, but I found some good "dissappointed" giraffe pictures too! Check out the mega furrowed giraffe brow on the second picture!
Source(s):
icanhazcheezburger.com
flickr.com

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The St. Louis Arch?
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I can't believe no one else has gone for this yet!

http://i29.tinypic.com/2ugnybl.jpg

This is my sad giraffe. I think I'll call him Franklin.
Franklin is sad because he just went to the tree and there were no leaves left on it, and Franklin is SOOO hungry.. Look at how little his tummy is... He even has tummy rubble lines! Franklin's even crying. Look at those big sad tears. He also appears to have some sort of hind leg tumor that can't be too comfortable. Maybe he's getting made fun of for it... Not a good day for Franklin.

Yep, Good ol' Franklin. He lives in my backyard. I just had to look out the window and take a picture with my camera, because that's a real photo y'know.. Maybe I'll bring Franklin a salad.
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This Giraffe was very disappointed when the drive-thru closed.
http://bblmedia.com/fat_giraffe.jpg

Dissappointed Giraffes might be look tasty to some.
http://i.dailymail.co.uk/i/pix/2009/01/18/article-1121222-0319B3BA000005DC-231_468x332.jpg
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Yes a giraffe who cannot get a boner
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here is a really pissed one
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I would assume it frowns. and looks down.
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When I read your question, I was assuming you were illustrating children's books or something of the sort.

A disappointed giraffe would defiantly be looking to the side, with it’s eyes rolled to the sky and it’s lips pursed yet slightly open in the center and gently curled up at the sides. Depending on how flexible a giraffes is, it would either be sitting with it’s legs rolled underneath it and it’s head resting on it’s back, or standing with it’s neck slightly hunched to the ground.

If you would like to know how an actual live disappointed giraffe would look, I can't help you...
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Don't they always look disappointed? maybe it's the lack of variation in their diet
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