ergophobe - 5:52 pm on Feb 2, 2012 (gmt 0)
I think the JQuery package plupload allows this and I believe also SWFTools.
I'm not certain on those, but I'm sure one or the other does the drag and drop uploading.
Of course, Wordpress offers this now and WP is open source, so you could peek in on how they do it.
As for the resizing, this is somethign you can do quite easily on the backend with GD (built into PHP) or ImageMagick (which runs as a binary on many *nix systems and can easily be installed) and is hooked in with a simple command-line syntax.
So pick one from column A, one from column B and I think you'll be there.