Since its teir 2 - You could use the FreeImage library (or DevIL instead). Both are cross platform c.
Ive used
freeImage (its great) and its really easy to load any format of image you like then use a single function to convert to 32Bpp then simply open a file, save 3 ints (width,height,32) and then save the raw bitdata.
That then gives you a file that can be loaded straight into a memblock which can then be converted to an image.
Thats really easy, - if thats what your trying to do.... Im guessing your probably looking at doing a lot more than that though.
Its possible to use these libraries to load any image type you like into AppGameKit too straight from the library into a memblock without going via a saved file if you wanted to support more formats or some obfuscated game data/graphics that people cant steal.