I started seeing some photographs that didn’t generate thumbnails. In Gallery, they’d show up as an aqua-colored picture frame with the top broken out of it. In Nautilus 18.104.22.168, they’d look like a cartoon photograph of a tree on the savanna with a setting sun. In Nautilus it seems to happen to JPEGs larger than 5 MB. Fewer photos are broken in Gallery.
In Gallery, I found a workaround by choosing “Edit Photo” and clicking “Crop.” This regenerates the thumbnail.
For Nautilus, there’s some discussion of broken thumbnails in 2.20, but I haven’t verified if this is the problem or not. Hmm, in Konqueror 4.2.4, the thumbnails for the same files are broken. And look at this–in Nautilus, there’s an option under Edit/Preferences, Preview tab, set to only show previews for files smaller than 5 MB. I bumped it up to 10 MB and, viola!, the thumbnails appeared. Konqueror has the same setting under Settings/Configure Konqueror, File Management/Views, Maximum file size. It also defaulted to 5 MB.