There are two major players in the web based image manipulation field, G-D and Imagemagick. Of the two Imagemagick gives you more options and is easier to code for. It generally requires that you have shell access. It's easiest to use either of them with php but there are other options there too. I almost exclusively use Imagemagick on my site.
Something you're definitely going to need is a good drag and drop script. Open the source of this web page, go to the very bottom of the source and get the code between and including the script tags and put it in the bottom of your page, or make a link to at the bottom of the page.
This is the best D & D script I've found.
Even though it may only involve a few pages, this is some pretty heavy coding you're in for.