Black jpg thumbnail problem

New to Mac's so I assume this is user error.

I Downloaded photos from my camera using iPhoto, The photos were shot in RAW + Jpg so two files for each shot. So far so good.

iPhoto doesn't support RAW, I know that, but Finder is still able to show a thumbnail which is useful.

The problem starts with some, not all, of the jpg's. They open/preview in iPhoto ok, but in Finder they just show as a plain black thumbnail, using Preview or looking at the details also gives a plain black image. Photoshop also opens them as plain black images.

If anyone has any ideas why this might have happened please let me know!

PS Screenshot attached if that helps.

Launch iPhoto holding down the option and command keys. A list of repair/rebuild options will appear. Try rebuilding the thumbnails and any of the other options you think would help.

Thanks, I didn't know you could do that, unfortunately it hasn't resolved the problem.

I've deleted all the black thumbnails now so I'll see what happens next time I download from the camera. Fingers crossed!