Actually pictures are the one thing that the iphone should be able to store like a USB drive (hopefully). If you plug the iphone into the PC you should be able to open your phone from mycomputer, and the only thing in there will be the pictures folder. You can drag stuff out of there and I believe you can also just drag and drop photos there as well (but only photos IIRC).
Failing that I personally think the best route would be to sign up for a free online storage thing like Dropbox. They give 2 gigs free, and you can install a small program that integrates your shared folder into My Documents.