We use Discourse very successfully as our outward facing forum. I am one of users that posts a lot of new messages. Recently (it is currently May 2016) I have found that I am getting a 403 message whenever I try to publish posts with included images or try to edit those posts to add an image.
At first I thought that the problem was with the links on a page as I can create a post if I leave out the images but then today it allowed me to post with an image embedded.
I use Snaggit to take a screen shot and copy paste the image into the post editor. I use an up to date Chrome. Discourse hosts the forum for us and I always accept the message saying to you want to use the latest version.
Anyone else come across this?