The oBot is definitely not a search engine; "different agenda" sums it up pretty well. Going by the information I dredged up last year (thread linked in first post) they seem to be involved in some pretty unappetizing filtering.
The noteworthy feature of the oBot is that its image-file requests aren't based on what is linked from the html page right now. It's what was linked from the page on its previous visit --even though that previous visit was over a year ago.
Independent of motive or agenda, that just seems inexplicable. It's like going out to buy something based on information in last year's Yellow Pages when you have the current version available.
Inevitable follow-up thought: Maybe it archives robots.txt along with the html file. So it's going by the rules of last year's robots.txt, not the current one :)
Confession: I have only now (after 16 months!) figured out why it was asking for one particular image file. It's because my front page used to use deep links to images in the assorted subdirectories; now it has duplicates in its own /images/ directory. D'oh!