It's a .DDS file so it should not make a difference.
Also, the stock foam looks horrible, in the main SH4 Mods Forum, look for a mod called JakeBox's Foam, much nicer, however it is a higher resolution. Try it and see if it slows down your rig.
Edit: Here
http://files.filefront.com/ROW+NEW+S.../fileinfo.html
RDP