This forum doesn't allow you to upload images, but it does allow you to link to images stored somewhere else on the Internet. So, you have to upload your media to a photo storage service somewhere and link to it.
When you create the channel, make it public. If it is not public, the public won't be able to see the picture. Upload media to the channel.
When you have uploaded the media AND if you have made the channel public, and when you click on a picture, you will see an area labeled "embed" on the right side of the window.
Select and copy the information within the "forum 1" text box.
Come to this forum and your message and paste that info directly into your post.