If the image format is not "jpg" then I'm sure they still can't display it on the forum because it's not supported.

Same image but different format.
1. https://i.imgur.com/s3vOUDs.jpg (BBCode ''forum'').
2. https://imgur.com/s3vOUDs (direct share).

The first URL can be displayed on the forum because the image format is correct, but not the second URL. Always make sure the image format that will be displayed on the forum is correct.