I've got a theatre prop, piece and its back wall serves as background for the "play" actors.  but when I try to put a jpeg photo of an illustration on it, the photo imports as too big and only a part of it shows above the stage. I've cropped the photo but how can I shrink it proportionally so it will fit better? 

drawn

This can be tricky. It will depend on how the wall is UV mapped. (that's the layout of how to fit a flat image on a nonflat surface)

Here is a cube with an image and you can see the image doesn't fit on one side.

If I change the scaling, it fits sort of but the ground area is in the middle of the face

that is because the UV map for this cube looks like this

Each of these grids is one side. This means one of these grids is for the side of the cube we're looking at. The image would have to fit in that grid.

so your poser setup would look like this

Now the theater's back wall would need its own UV map. I don't know what that looks like and looking at the read me, it doesn't list templates being included. You might be able to open the back wall's current texture in Photoshop (or other 2d software) and use that as the template if you can figure out what section the wall is. Then save that as a new image and apply it.