I have experienced this issue both in Windows 7 and Vista with Odd images, (this actually relates to explorer rather than Photo viewers), but the issue may
In my case it was due to a corruption in the Horizontal and Vertical DPI resolution of the images - If you open the image in Irfanview ( IrfanView - Official Homepage - one of the most popular viewers worldwide
), the Info option provides an editable entry for the DPI setting - make sure that these settings are equal, (eg. 100x100, 50x50), and they should then display correctly.
Always do this on a copy of the image just in case