I assume you already have the images for the pictures.

You can make a box model which is thin like a picture and use the image as its texture. Here's an example of a model with textures.

If you don't want the pictures to move, you can pin it to the wall by setting the model's <pose> and making the model <static>.

I recommend you take a look at this tutorial before anything:

http://gazebosim.org/tutorials?tut=build_robot&cat=build_robot