I just wrote a Ruby script that does pretty much the same thing: https://gist.github.com/865841. I used a simple bloom filter for de-duplication. It's a bit overkill. A simple hash on the twitpic_id will probably work just as well if not better.

I see lots of photos of someone's passport or wallet and some pamphlets. Some celeb photos, emo shots, a cafeteria. No earthquake footage.

True, right now any retweets are shown as duplicates.