I do my posters in Photoshop (obviously) and then go to PhotoBucket.com to upload them.
Forgive me if I'm going into too much detail about basics which you may already know... After I import the photos, there is button there that says "Generate HTML and IMG codes" and I push it. It gives me code selections and I choose the IMG code for message boards. Then paste that code into the post.
I imagine it's the same with Flickr... You'll have to upload the photo to a host and have them generate a IMG code for you to use.