Sound Images was a British company created by Tony Williams and Paul Tonge. They started doing game audio around 1989 to about 1994. While they primarily composed for computers, they would occasionally write for home consoles as well. The company not only dealt with music composition and sound design, but also took on the arduous task of converting other composers' songs to another platform, usually with lesser audio capabilities. Paul later left the company for Software Creations (who contracted Sound Images for sound work on the Genesis), leaving Tony as the sole composer.
Tony Williams wrote a sound driver in Z80 assembly. Music was most likely written in Notator for the Atari ST, and then ran through a utility to convert MIDI instructions to assembly machine code for the driver to read.
Note: These are all of the games that credit "Sound Images" for music and/or sound. To see who specifically worked on the audio, see Tony Williams or Paul Tonge. Also, games with Composer? denote games on MobyGames that list Sound Images, but don't actually credit them in-game, or only credit Tony or Paul.