Yeah like everyone else said, they need to be textures within a WAD.
Use a trigger_camera to quickly take over the player's view and point at a textured wall. If the resolution is too low, split the image into multiple textures, and use 4 textures instead of 1.
Other than that, you'll need to make a custom overlay and code it into the gameplay - which is far more time consuming and then you'll need to distribute the custom binaries.