Image Steganography

You'll have to distinguish between pixel-based (Bitmap) and palette-based formats (GIF) for which the steganographic technique is quite different. Also be aware that there are image formats like JPG that lose information in the compression process.

I'd also advice some general introduction to steganography including different formats.

This heavily depends on the way the particular image format works. You'll need to dive into the internals of the format you want to use.

For JPEG, you could fiddle with the last bits of the DCT coefficients for each block.

For palette-based files (GIFs, and some PNGs), you could add extra colours to the palette that look identical to the existing ones, and encode information based on which one you use.