I am sorry I cannot be of any more help, as you say it might be a vista thing.
It might be the files themselves, as as you say 90% of the files are fine?
All I can suggest is that you save them as a gif file, as a png file has higher file guality.
Again, I am sorry that I cannot be of anymore help but I think that it is just one of those things in vista!
Kind regards
Jamie