To scale the image, make sure the GE image you take has the Scale Legend turned on. Then all you have to do is measure that in pixels and do the math. I usually take the time to determine the altitude I want, and then snap all the images at the same altitude.
On my monitor the scale is always 230 pixels, but that changes depending on the monitor and resolution. Doing the math, 191 meters/230 pixels equals .8304 meters/pixel, so I could zoom out a bit. Normally I zoom out so that the scale is 460 meters, so each pixel is 2 meters. That way I don't have to take as many snap shots. Then I rescale the final image in Photoshop.