Sorry, modifying the Windows XP Welcome screen isn't supported. I had tried it originally, but you need to do more than just change the image to make it look good. There are other details, like lines and gaps that need to be modified as well. If it was just a straight-up image thing it would be supported, but sadly that's not the case.
It is a grand idea and it appeals to the artist in me (I display my photographs as wallpaper).
