Closed group sharing alongside sharing within Gnutella won't be in for some time (except if a Java coder comes along who wants to do it him-/herself
). There are quite many complications coming with that, and we'd need someone to really tackle them all.
Getting thumbnails of images would be possible, if most clients where Phex', which isn't the current case in Gnutella. But still, it could be done with a search reply extension. The hard thing about it would be to make sure that it can't be misused to attack your computer, and to create the previews automatically for any files. Using PNGs and finding the correct libraries to decode the files could solve that...