DON'T PRESS THE ADD POST BUTTON YET!
Look underneath your newly presented posting and you will see the text box again, ready filled in, so that you can edit the message or the gif address. You can then tick the preview box again and repeat the process over and over until it works properly.
Once you have it working, you then press the 'Add Reply' button (WITHOUT ticking the preview box) and the message will finally be posted to the board.
This info has now been incorporated in the Board Admin forum along with complete instructions on how to include pics in your postings (based on Wendy's description...Thanks Wendy!)