I think I've seen this happen with ljpics too, and I also think I get what is going on at a conceptual level (couldn't retrieve the image--maybe within a time limit, possibly because of a memcache miss?--so instead its alt= is dumped in the same space), but is this actually indicative of something wrong with the image (the larger versions, also reduced from the original, render just fine), or something wrong with fotobilder that it couldn't process the image correctly for the thumbnail size?
For instance, see the fourth image ("Lubricate." Relax, it's about putting olive oil and butter in a cooking skillet.) in this gallery.
Update: Please note that the image will display correctly now, as I deleted and re-uploaded it in response to a comment below. There's a screenshot of what I saw before included in the comments.