GrafX2 is a bitmap paint program

GrafX2 is a bitmap paint program inspired by the Amiga programs ​Deluxe Paint and Brilliance. Specialized in 256-color drawing, it includes a very large number of tools and effects that make it particularly suitable for pixel art, game graphics, and generally any detailed graphics painted with a mouse. The program is mostly developed on Haiku, Linux and Windows, but is also portable on many other platforms.
Main supported platforms are:

• Windows: 2.6 Installer / 2.6 Portable / Nightly build (latest changes, may be unstable) 
• Haiku: pkgman install grafx2 
• Mac OS: 2.6 for amd64 or PowerPC 
• Linux: Nightly builds

You can download it from the official website.
One good tutorial about how to create Blending and Transparency with this tool can be found here:
If you want to use for your game development then this is a good video tutorial about Tile Editing
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