When you're in the editing page (the one where you usually type your story in order for it to be posted later), there's this "Source" button on the top left on the input (typing) box. If you can't find it- it's a bit to the left of the button that bolds the text.
Once you click on the "Source" button, you can put in HTML codes, and it will work. When you're done, just press the same button again, and you'll see the results of your HTML coding.
And about HMTL code itself... I'm not good at it, sorry :P

You use a link. Find a picture you want to use and when you go to edit the forward of your story paste the link in the box. There's two different boxes one for the poster and one for the background image. People that use graphic shops usually upload their pictures to photobucket and paste the image into AFF and when you right click and open the image in a new tab the link is on that page. Use that link in the boxes and you'll have a poster or background :3

(If you weren't talking about linking and background images then I don't know what you mean)
If you meant editing one poster to look fancy then ask someone from a graphic shop. (I'm pretty sure most of them use photoshop)