Gratis software to edit 32 bit BMP files (ARGB Bitmap)

Question

Microsoft MFC Ribbons use 32 bit BMP files for the transparent icons. While my favorite pixel editor, Paint.NET, opens such files, it displays the transparent pixels as black instead of the alpha channel.

As mentioned by Microsoft themselves, I find the following to be true:

Many image editing tools do not preserve the highest order 8-bit alpha channel when loading or saving 32 BPP images.

Therefore I am looking for:

• a software for Windows (10 and 11 at least)
• which is gratis (Open Source preferred)
• to load 32 bit BMP files
• supports alpha channel
• just basic editing tools needed, similar to Microsoft Paint
  • pencil
  • fill bucket
  • some selection tools
  • basic shapes (lines, circle, rectangle)

I have tried:

• Paint.NET. It shows the transparent pixels as black.
• GIMP. It shows the transparent pixels as black.
• Microsoft Paint. It shows the transparent pixels as black.
• Affinity Photo, which works, but is not gratis.
• BMP Image Editor, but it can't even load ARGB images.

Answer 1

I found PixelFormer by Qualibytes.

a software for Windows (10 and 11 at least)

It claims to be compatible with Windows XP, Windows Vista, Windows 7, Windows 8, Windows 10. Both 32-bit and 64-bit versions.

which is gratis (Open Source preferred)

The beta version (0.9.6.3 RC3) is available for free. Not Open Source, though.

to load 32 bit BMP files

That's one of the first features mentioned: When using Pixelformer, you can always work in the 32-bit RGBA color space.

basic editing tools: pencil, fill bucket, some selection tools, basic shapes (lines, circle, rectangle)

It has pencil and brush, fill bucket and gradient, rectangle and magic selection, rectangle and circle, as well as text.