Under some circumstances, Shotwell shows no photo thumbnails while importing. I was trying to download some pictures from my camera (Canon EOS 400D) with Shotwell and the program didn't show any preview for the pictures before downloading them (it just shows a "forbidden" snapshot). After importing them everything seems fine.
I've found this bug report but no solution is given for now. It seems there could be a solution for EOS cameras, according to the last comment. This guy links to a patch, but it's not downloadable. Any ideas of where to obtain a deb package of this patch or of any other solution?