there is an option to automatically resize images that are larger than the allowed size, in the admincp either in "vbulletin options" ...... i cant remember what the exact option group is though.
This way, when a user uploads a file that is 250kb, and 400x400, and your allowed size is set to 20kb and 250x250, vbulletin will automatically resize the users image to fit those parameters.
Which saves YOU a TON of time having to mess with download, resizing, reuploading etc.....
scratch this task off your list of things to do, and use the time for something else.
welcome to the wonderful world of vbulletin