Hi,
There is the Adsense mediapartner bot, which is browsing pages on which adsense ads are showing (or supposed to show). This bot is the one which extracts the content of the page, to select which ad to show,... at least this was the initial idea, at the time where all ads were contextual only ...
By the way, from time to time, I see hits from mediapartner, the right UA, from the right IP range, downloading images from my site, independently of pages it visits. So I am wondering, why is that?
Is it to verify that images are not against Adsense's rules (nudity, etc...) ?
or/and
Does Adsense can exploit information from images, in order to select ads? If so, I am wondering which kind of information, the Adsense bot can be looking for. May be it can identify image similarities, ... for example, a photo of a shoes, might lead in displaying shoes related ads...
I wonder at which point the Adsense bot is sophisticated ... but still it's too bad that Adsense does not make use of the GoogleBot (search).
edit: just realized this is my 1000th post.