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Matt, Image scale simply means how large the object (a planet for example) is projected on to a cameras CCD or film plane. In other words, how large the image appears. Sort of like a slide projector close to the screen produces a smaller picture and as you move it further away the picture gets larger.
The shorter the focal length the smaller the image and vias versa. There is another consideration that affects image scale even if the focal length is fixed and that is the size of the CCD chip in the camera. For example a camera with a 1/2 inch CCD attached to an F/10 scope produced an image slightly smaller than and camera with a 1/3 inch CCD. On the same f/10 scope, a camera with a 1/4 inch CCD produces an image larger than the 1/3 inch CCD camera does.
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