This tool for addon authors applies the Addon-On Pack badge to an uploaded image.
The restrictions and requirements of the uploaded image are...

• The dimensions must be between 661 x 661 (badge size) and 4096 x 2160 (4K).
  1920 x 1080 is recommended.
• The file size cannot be larger than 8 MB.
• Common image types are accepted.
  JPG or PNG is recommended.
• Animated images will not work.

The uploaded image and generated image is not saved on the server, so download it immediately.