You could just set the quad with the image to the size you want to use and change the position and scaling under the texture cords tab (not transform) until it's cropped how you want it.
If you do want to mask it you need to make a material and then set the blending mode to alpha masks. Then apply it to an object such as a quad and position it where you want to cut the image. To see how the mask is correctly affecting your scene you'll need to place the object you want to mask and the object with the mask material inside a layer object.
#XPression