IVan - the main "problem" is file size.  As long as the image file is within acceptable limits, I don't think we'll quibble about format.  Of course, the requirement to keep it as a jpg or gif is more because of the universaility of these two formats, but with practically all graphics/imaging programs and browswers capable of handling the PNG format I see no significant problem.

One thing the I've seen, though, when preparing an image for uplaoding to Simviation, I usually go through Paint Shop Pro's "export" tool and preview several of the formats - JPG with various compression levels, PNG and GIFs.  I've found that not always does PNG  give the "best" file size/compression....

Again, as long as the image is within limits (and viewable by most folks), I don't see a problem with a different format.