There’re some libs out there which can be used for develop homebrew software for the Wii, without doubts the most used is libogc, which is included in devkitpro’s releases. Although that is a very completed lib, you’ll need some other libs to do stuff which is impossible with only libogc. For example to load images or sounds.

PNGU is lib that ‘wrap’ a port of the famous libpng to the Nintento Wii, presented as an alternative to libjpeg (which has been recently ported to Wii too). PNG format has some properties which make it interesting a transparency color, alpha channels, it doesn’t modify the colors (as jpeg do when compression), open format,… There’s a set of functions (PNGU_*) which let you load a png to the buffer easily