Yes, image size should always be smaller than screen size.

Oh boy, I crack myself up.
OK, seriously. For us we always make sure we have a 2 inch gutter on all 4 sides. This allows for the tight screen tension found close to the 4 sides of the frame which are unusable.
So for a 20X24 screen with a 1.5 inch frame wall means your inner screen size is 17X21 less your 2 inch gutter leaves an image size no larger than 13X17.