If running on Windows or IRIX you can use the Image Format Library from SGI. It supports numerous formats (e.g. PNG, JPG, TIFF, GIF, SGI, etc.). I wish there was a version for Linux.
I also have a the source to load the SGI formats (e.g. INT, INTA, RGB, RGBA) if interested. It was extracted/extended from Brian Paul’s Mesa 3D library.
Last I tried to build OpenIL, it was totally broken on the Macintosh (ie. it was going to take several hours work to fix it even to compile; not an effort I wanted to make when QuickTime just works). Is the situation any better these days?