Tropical Freeze Textures: the TXTR format. RTB and Parax pretty much figured it out.

[Kold-Virus]

It's using a custom type of LZSS compression, which the info can be found here: www.metroid2002.com/retromodding/wiki/LZSS_Compression

Trying to decompress it

Best of luck, my friend.

I also looked at a GFXL file I found in UIPak.pak. Images seemed to be in a compressed TGA format.