mosaic is a specialized image editor.
Unlike a traditional editor, mosaic uses user-defined logic scripts to decode images in virtually any uncompressed format from inside of an existing file, which it then exports as a bitmap.
After editing the bitmap in one's graphical editor of choice, mosaic can then import the bitmap back into the file it was exported with, using the same logic script.
This fills the role of tile editors in ROM hacking (for instance, editing fonts and sprites), but can also be used for much more due to its flexibility.
mosaic v07 (Windows binary and source code)
mosaic is released under the ISC license.