In a way, they already did: Posterous. Fortunately Posterous is more blogging/photoblogging than photosharing. Sadly that looks like a pure talent acquisition and there are hints that the product will be killed. I hope not. Nobody wants a quick-blogging world dominated by Tumblr.

Getting into pure photo sharing would trigger an arm's race that Twitter cannot hope to win, and one that would distract it from its stronghold... the heart of the stream. Despite being weakened by Facebook and Google+ in this domain. Twitter remains what Om Malik calls the 'message bus of Web 2.0.'

The smart direction for Twitter is to buy up some deep Analytics capabilities downstream of its own firehose.

In pictures, a specialized subset, like those meme images with slogans that spread virally on Facebook, might be a good direction. They are like visual tweets. That's actually a bit of a threat, since they stand out from 140 char strings but are not directly parseable.

But all this is moot. We haven't seen twitter do any big, bold things in a while.