Custom texture format?

I want to write a program that will let me load any texture file, and then save it plus some other data to a custom file format, then I want to make a sister library for use in my code just for loading this format.

Im going to use DevIL for loading the textures (because it supports just about every format there is), my only experence with writeing files is with fstreams useing whitespaces between each seperate value in a file, I would not want to do that here because it would bloat the files. Any suggustions on how I should do this, or any other part for that matter?

You'd want to use binary read/writes - ifstream::read and ofstream::write or the C stdio versions. Some operating systems provide functions to read/write in binary that are often more efficient than the standard C/C++ equivalents.