Instead of showing what the new values will look like, why not show them as additions, subtractions, etc of the current values? This way it is much easier to read what is being done
Add grid lines so users can better estimate the inputs the should use
Don't crop the images, just overlay 'mattes' or something so you can still kinda see what's on the borders
This would allow for the specification of a single 'width' or 'height' argument. This is handy since the aspect ratio of the subject is fixed, and often (unless you're very lucky!) you'll just end up with useless white bars on the dimension that has aspect.