I'm currently in the works of doing something similar, and right now, the best I can do is point you to some stuff here on the forums.
http://www.pligg.com/forum/showthread.php?t=2443 - This thread here does give some good quick pointers on the moving and thumbnailing code. Theoretically it should work by setting the source as the url entered, and starting from there. Not really much of a tutorial, but if you have some coding skills, its better than nothing. When I get things working on my site, I'll make the changes available, since others seem to want to do similar things, plus its always nice to help the community that has helped me.