Thank you both for your replies. I really thought that I was missing something, and that the frame information was hidden somewhere! I'm not really into game art culture, but I get the sense that lots of artists who make animations that are uploaded to this site do so just for the fun of making something that looks good -- but surely the chance of their work being used in a game is always on the cards (the site is called openGAMEart.org, after all).

In the Blob example that I used, the frames seem to be arranged in a way similar to the output from a sprite/texture packer that I used briefly a few years ago... and I'm pretty sure that that program output a sprite sheet as well as a text file with frame information. Maybe the artists are using programs that generate this frame information, but they're just not bothering to upload it, because they don't realise how useful it would be to game developers. If there's an FAQ here for uploading guidelines, maybe it would be good to add a note on this.

At the risk of being flamed for cross-posting, I might post a similar question in one of the forums frequented by wielders of the paint brush, to see if they're aware of the pain that we have reverse-engineering their animations.