When you have the thumbnail sized picture up in photobucket, click 'edit' above it, and you then have options to reduce the size. As that's so big I'd do a 75% reduction to start off with and then take it down in 25%'s from there. Photobucket does take a few minutes to 'catch up' though so do click refresh a few times and wait for it to resize before you manually resize it again - otherwise all of a sudden you will have a dot instead of a photo :lol:
Either that or on your own pc, open up paint or photobucket and resize the picture there, then upload it to photobucket when it's already the right size.