What program do you use to make the image? Some programs make special things to the bitmap images and that could result in UTAU not being able to use them.
Try to open the image in paint (start, programs, accesories, Paint) and save (better if you "save as..." with a different name and use the new file instead of the old one) it again from there.
If that still doesn't work, open the image with paint, select all, copy, create a new blank file 100x100, paste and save with a different name.