Typically uploaders can choose to upload in any of our supported image formats. Some users, though, choose to upload uncompressed images when functionally-identical lossless formats are also available. This wastes a tremendous amount[0] of storage and backup space.
Suggestions, in order (I think) from easiest to hardest:
- document guidelines about preferred vs. hated commons image formats
- reject uncompressed files at upload time
- compress new files at upload time
- actively detect and replace uncompressed files with lossless-compressed equivalents
