GIMP Sprite Plugin
There's a great third party project which now includes a GIMP plugin for Windows or Linux. This lets you edit sprite files using this famous free paint program.
For the harder core, there is also the source code to a general purpose sprite file library, and command line conversion tools.
Go to the project web site.
Creatures Sprites 0.2.1 (LINUX)
Creatures Sprites 0.2.1 (WINDOWS)
GTK Scriptorium v1.0 (LINUX)
easyPRAY v0.4.0 (WINDOWS)
easyPRAY v0.4.0 (LINUX)
C16 Tools v0.3 (WINDOWS)
C16 Tools v0.3 (LINUX)