Brian

There may be something I'm not thinking about, but I don't think you're going to be able to have a simple tag like that with just the imageid without either doing a good bit of coding/hacking, or having a query run for each image (which I personally wouldn't recommend). If some code was written to first scan all the posts within a thread for the [GALLERY] tag and the imageids grabbed then the thumbnail and other info could be grabbed in one query and thrown into a cache, but that would still be a little tricky and maybe overkill.
One alternative that would be fairly simple though and should only require a template change would be to just have the tag look something more like this:
[GALLERY=imageidhere]thumbnamehere[/GALLERY]

Not quite as easy to use as Brinnie's idea, but would still be easy for users to copy/paste from the showimage page.