Seems like the source has been scaled in the scene?
When you click on the scene and hover over the different sources, where does the lines of the bounding box for the camera appear? If it's outside the frame, all you need to do is click on it and rescale the source so it fits within the frame, then you're all done!
In the attachment below, you'll see that I've scaled the background image so the overflow from the odd dimensions of the image ends up above and below the output frame.