LibreSprite is a nice free one that's forked from Aseprite. Source: over 1 year ago

LibreSprite is an older version (a fork) that has downloads. Source: almost 2 years ago

I have also seen Aseprite being recommended. It looks nice and I'm sure it works well, but it is not open source unfortunately. There is an open source alternative called LibreSprite, which seems to mimic Aseprite quite closely. Source: over 2 years ago

If this is only meant to make an existing game icon a Big Sur compatible one, https://img2icnsapp.com can do that automatically by dragging and dropping the icon into a template and saving it as an .icns file…. Source: 7 months ago

There is this simple app that lets you do that, but the folder option is behind a paywall If you don't want to buy it, I could try making it for you once I get home. Source: about 1 year ago

Thanks for providing the customizer! I like the logo but immediately made the background darker to increase visibility on my Mac dock. I used image2icon in order to convert the provided .png file to .icns, otherwise it wouldn't size correctly for the dock to match other icons. Source: about 1 year ago

Image2Icon (freemium) - converts normal pictures to the .icns format of macOS to set them as icons for apps, folders, etc. Brew install image2icon, Mac App Store. Source: over 2 years ago

If you want to change custom folder icon more effectively, I recommend this tool: https://img2icnsapp.com. Source: over 2 years ago