What interesting for us is that there are a number of different tools on the site that can help get to the bit depth that will be accepted by various services.
- Riot - lets you cut down to any number of colors - https://www.portablefreeware.com/index.php?id=1821
- LazPaint - includes color depth options on save - https://www.portablefreeware.com/index.php?id=2625
- GIMP - if there isn't functionality within the program there's probably a plugin that will do it - https://www.portablefreeware.com/index.php?id=644