I set a project to use the IS Widetheme for a Basic MSI project.
I copied the original images into one folder. I then made modifications to those images, using the same pixel size and color depth and format, then stored in a second folder.
In a script, I'll swap out the images, then call IS to build the project, then swap the images back.
Under Windows 2000 and XP, the images do not appear and I see just the grey dialog.
My only thought is the encoding of the images might not be supported by the libraries on the older versions of Windows. Try, for example, changing your images to BMP or otherwise resaving with Windows Paint, and see if that changes the results.
