This may help -
https://www.eqinterface.com/forums/showthread.php?t=458
It doesn't mention it in the post, either it wasn't supported at the time or they didn't know, but a .png with transparency will also work in a ui. If you decide to use .tga graphics you'll want to study how transparency works for that type of file as photoshop handles it very slightly different than most other formats.