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If you are really sure your site is being spidered by this user then it looks like they are trying to do it stealth-ily - faking a user-agent is an easy thing to do, detecting a fake user-agent is a whole heap harder.
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Given what your site does I'd say someone with only a fraction of a clue has figured out that they might be able to get the images and has gotten hold of a download program.
Their first step was the copy their own browsers user-agent (or use a fake one that came with the program so that you might not notice their tricks or at least would have a harder time spotting them.
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p.s. Nice owl :)
- Tony