Go to the pic on the bucket, click on it, when it opens up put curser on the pic and a menu will drop down click on edit and then on resize, from there you can resize, make sure that keep proportions is checked and then use the slider to get the size you want.
Also on the upload screen, click on Custonize your uplaods options and check 640x480, every thing that you upload should come in at that size.
Ive never uploaded any thing smaller than 640x480 so not sure if it will increase size to their site but you would think it would, give it a try
Im not a photobucket guru but it should work.