List of 15 Image manager apps, although not strictly web some of them could be useful and offer batch resize and format conversion

http://www.techlicious.tv/2007/12/16-great-media-managers-to-organize.html

I'd also add Easy Thumbnails to the list, its a great free too for batch resizing images and also adds an option to your explorer menu to repeat the last used settings on a file without having to open the app.

http://www.fookes.com/ezthumbs/