Not sure to be honest. I haven't taken the dive into Vista yet... :giveup:
Just shooting in the dark here, but since .tga's are natively 32-bit files, is his system's color depth settings/resolution set high enough? I would ony assume so since I've heard Vista has alot of eye-candy, but it's something he might want to check.
Also, .tga are tricky to begin with since while the extension is the same, there's at least 3 different (separate) signatures that can be used to create them. Vista may or may not support all 3+ signatures. One "fix" might be to download a program that adds the signature recognition patterns to the system.
Another culprit could be Vista's Aero gui. Is his laptop capable of using it? If not, this may cause problems with recognizing certain .tga files (because of the alpha/transparency channels). Not really sure if there's a fix for this other than enabling Aero.. but you can try (at your own risk ;) ) a few 3rd party apps such as:
IrfanView (
http://www.tucows.com/preview/194967)
TGA Tool (
http://www.softpedia.com/progDownload/TGA-Tool-Download-30526.html)
Thumb Plug TGA (
http://greggman.com/pages/thumbplug_tga.htm%7Cpagenum=2%7C?q)
While Photoshop CS2 would be a good option, it's not necessarily within the "fun-money" expense range... Maybe perhaps ask your cousin to download something like the free version of Quicktime (
http://www.apple.com/quicktime/download/win.html) (which supports all signatures). No need to get the Pro version, the free one should work.
Also, most mods that involve skinning tend to make skins for the "high" resolution settings (512x512 template), so make sure your cousin has his game graphic settings set to such a level (at least for textures) :)
Of course the standard update video drivers, blah blah, etc. applies too.. hehehe