These are the requirements to be able to have images in Forum messages

1. The Support Center feature has to be enabled
2. The image you want to add to a forum message should be added in the Documents tab in the particular project. Check the Support Center flag for the image and copy the link.
   
3. Then when creating a new forum message create a "img" tag and paste in the link.

If you have some trouble with step 3 above. You should use the menu that looks like an oak (right to the anchor icon) and paste the link to the image there instead.

Sample image!