lucy24 - 7:40 am on Jun 26, 2011 (gmt 0)
.* means "there might be some stuff here". Since the rule is only concerned with the extension, it doesn't matter what comes before it.
You don't need to list all possible extensions, just the ones you've actually got. The main three are .gif, .jpe?g and .png. If you really have .bmp format (Windows bitmap?) you should add it, just as you might add .pict or .svg or .jp2 or anything else you've got lurking in the directory. But if they're banners they probably all use the same extension anyway.
To be safe, add an NC to the rule, in case someone asks for JPG.