Why wont some xp icons become thumbnails

[Knobby_14]

In " My Pictures" folder in XP, I click on show thumbnails, and 98% of the icon show as thunbnails, but some just stay as icons, why is this ?

 

[Hawkeye22]

Not all images provide a thumbnail, thus the icon. Some third party programs can display previews/thumbnails even when they don't exist.

http://www.freewaregenius.com/add-thumbnails-to-all-your-media-files-in-windows-with-media-preview/