BmpToZlibGC 1.1 is a Windows software to use for Gamecube/Wii homebrew development.

Quote:

This application was original written by PaceMaker in 2007, and mimicked the GFX2GC code, but with open source and an extra transparency option. I’ve converted Bmp2GC to use the Zlib compression scheme seen in many of the Wiibrew emulators, including Snes9x-GX. By using the compress() and uncompress(), a developer can include large images into their elf files without having to bloat the program. Compression is also largely based on the complexity of the image, so large colored areas will decrease size, along with file sizes.

http://www.wiibrew.org/wiki/BmpToZlibGC