SteveS,

I think you'll find RIFF thumbnails show up if they're saved from Painter X or Painter X.1 and you're looking in the folder where they were saved.

Works for me in WinXP Home SP2 using Painter X.1 and as far as I can tell, any RIFF files saved in Painter X also show up as thumbnails (not only in the Open dialog upper right corner where they've always appeared).