Hi, Read: https://www.lifewire.com/how-to-fix-a-400-bad-request-error-2617988

Point 6) says:

If you're uploading a file to the website when you see the error, chances are the 400 Bad Request error is due to the file being too large, and so the server rejects it.

Check:

1. If you don't have limit of files on server. If yes, you have to delete not used files. For example uninstall not used templates or components
2. Check file name, no extra marks
3. Try to upload image that have max 5Kb jpg (1st) / png (2nd) - for test only. Save files and use them.