If the filesize is too large, optimize (if a BMP or PNG, try saving to JPG) or resize it. Windows' own "Paint"(brush) can resize the images, GIMP or Photoshop can specifically optimize them to not lose too much detail, but sometimes dramatically lower the filesize.
The forum itself has a limit of 4Megabytes per upload, which should see you into 4K images if optimized. It is possible there is another server configuration point that has lowered the limit more.
OK, not surprisingly a 9Mb JPG was rejected
A 1.8Mb JPG has uploaded
Another one at 2.5Mb was too large, so you may need to keep images below 2Mb then.