ID3Pic is a Command LIne Tool for adding (and eventually replacing) album cover images to files with ID3 Metadata tags, like MP3.
I wanted a tool to put album covers on my tunes. A tool that I used to use would sometimes clobber a bit of audio data at the start, which would take away the punch of a kick drum when there was a kick at the beginning. There exist plenty of other actual tools that work properly, but I just wanted to do something interesting with file I/O in C. There exist plenty of ID3 libraries, but that would defeat the purpose of me making something from scratch.
For Windows: make
For Linux: make linux
I developed this on a Windows 7 machine with Cygwin gcc, meaning it mandates old C89 style C. Cygwin is terrible.
./id3pic <MP3_AUDIO_FILE> <JPG_OR_PNG_IMAGE_FILE>
If you input something like BigBass.mp3 and AlbumCover.png, a new file named BigBass_out.mp3 will be produced with your cover photo attached to it. Hurray!
ID3 is a format for metadata tags used in audio files. An ID3 tag contains one or more frames where each frame has a chunk of metadata. This is where items like 'Contributing artists' and 'Title' are stored, which you've probably seen on your MP3s, each of which are stored in a certain type of frame. It's also where album covers are stored, specifically in an 'APIC' frame. The current versions that are more or less in use today are 3 and 4, ID3v2.3 and ID3v2.4. Currently this tool supports both these versions and complies with whatever the input file has. FL Studio 12 (probably FL 20 too, haven't updated because lazy...?) currently outputs with version 3.