There's a relatively low priced DLL available from http://www.smalleranimals.com/ that does thumbnailing amongst many other things. The docs for it aren't great though, and I'm not sure if it's compatible with Blitz.
I think it was called the 'ImgSource' library. You can also get a free demo version of it.
I think it was called the 'ImgSource' library. You can also get a free demo version of it.