Hey there! First off, welcome to Dokuga.
In order to post challenge banners (if that is what you mean) inside a challenge thread, you need to use the Link Button. You should be able to see a small icon that looks like a world globe inside a box, with a chain link in front of it, at the end of your Boardcode: space.
Clicking that will bring up a little box that says Size: with an empty box. You can put in the size, or leave this blank. (I usually leave it blank). Once you do that, there is URL: with a box as well. Simply add the link for the image into this box, and then click insert.
It will insert itself wherever you last stopped typing inside the Message:
