PNG is one of my favourite file formats. It also has a chunking system which lets you store extra data, which isn't really covered here. Definitely worth looking into if you're interested in binary format design.

The other image format that's worth knowing is TIFF, because it's an insanely simple and flexible format to write out if you don't have access to a library. It lets you re-order the image data by tiles or scan lines, and lets you put various tables almost anywhere in the file, which makes it great for outputting large images from a parallel renderer: you can chop it up however is suitable for the algorithm, write out the parts to disk as you get them, and then write out a table at the end describing the order at the end of the file, without having to seek.