It could either be a problem with your image files (which could be difficult to solve) or simply a problem with the Photo Gallery program. Since it's total rubbish anyway, the first thing to do would be to install a far, far better program and see if you can open your images with it:
http://www.irfanview.com/
(When installing it simply accept the default settings regarding the type of files it opens. Then double-clicking on an image file should open it in the program automatically [or open Irfanview first and go via File > Open]. To advance through images in a folder, press the space bar or the Page Down key)