If you load your images into ImageShack at
http://www.imageshack.us they provide auto-sizing. Kind of cool. Though I have personal pics on another photo-sharing site for sharing with friends & family, I use ImageShack to post pics for use at forums.
Imageshack apparently even have a tool where you drag a photo from your Windows Explorer dialog box into the forum text entry box and it will upload the pic to ImageShack and insert the appropriate code into the forum text entry box. I haven't tried it yet though.
Even without the slick drag & drop tool I like that ImageShack auto-sizes the files to 640x480 (it also has other sizes if desired).
Ken K.