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If you or someone you know owns one, you could just take pictures of it. Otherwise, try looking at some royalty-free stock photo websites.

We have the computers but I don't have the talent to get a picture of an iMac that looks like it's floating in a white background :eek:
 
Take it against a single-color background - use a well-lit cardboard box or white wall, or something. It'll make it easier to use Photoshop or something similar to cut out the background and paste the iMac onto whatever background (or transparent background) you like.
 
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