1.) Make your poster. Click on the background layer (First Layer)

2.) Select > All

3.) Select > Rounded Rectangle. Radius: 20. (or however round you want it.)

4.) Select > Invert 

5.) Layer > Transparency > Add Alpha Channel 

6.) Select your eraser tool, erase the edges of the poster. 

7.) Finished Product. (:

*Note, IF YOU HAVE ANY TEXTURES ETC ON YOUR POSTER MAKE SURE TO CLICK ON THOSE LAYERS AND ERASE THOSE PARTS AS WELL. (: