Yes, I've used it for images as well and it seems to work fine. But Google's help section doesn't show any examples of a wildcard being used after the file extension. It seems like it would work fine, but I'd like to know for sure before putting it up.
Patterns must begin with / because robots.txt patterns always match absolute URLs. * matches zero or more of any character. $ at the end of a pattern matches the end of the URL; elsewhere $ matches itself. * at the end of a pattern is redundant, because robots.txt patterns always match any URL which begins with the pattern.