I have exactly this problem. There is no gallery locker on the phone. I have removed the thumbnails from the DCIM directory, and Android has re-scanned the SD card and created three sets of thumbnails: Camera, Whatsapp and Wahtsappvid. But when I view them in Gallery, I get the same symptoms as above, with each thumbnail showing as above. If I click on one of these 'blank' thumbnails, the picture displays ok. If I use MediaSee it seems to display all the pictures as thumbnails.
Perhaps of interest may be that the picture above looks like a Motorola Defy Mini phone, which is what I have got. Sorry if this is the wrong way to ask this question, but this is the only post I have seen which shows the problem I have got.