I think the most common way to add posters in TF2 Hammer is with the Overlay tool. However despite posters being flat, this method may make your posters appear "too flat" for the surface of walls and such.
Meanwhile TF2 has props for posters. So I figured I'd copy them and reconfigure them to make them reskinnable and accessable for me and others to use. Being flat poster props they are the most easy to reskin.
There are five props in total, each based on poster-props from TF2. They are very easy to reskin, even for novice and I even included .png templates. Each prop has six skins in total (default, a to e) and an instructions.txt for how to install and reskin. (Basically just save over the existing .vtf files)
This is my first upload of props and I hope they will be very useful to others. As they are default TF2 props, no need to credit me.
P.S. Couldn't find the poster layer that makes the Update Posters appear worn, creased, dirty and signs of tape and such. If I had it, would of included.