I would think you could open them with a photo editing program like XnView,
Photoshop, Paint Shop Pro, etc. Then resize them and save them with a
different file name as the larger size. You can also use your own picture if
you want more choices than the default pics. The default pics are all in
..bmp format, so I do not know if that's the only picture format that can be
used on the welcome screen or if you can use jpg, gif, png, ico, etc.
You can find the welcome screen pictures at C:\Documents and Settings\All
Users\Application Data\Microsoft\User Account Pictures
