Hi Corey,
Add in the actual model dimensions, (listed in dialog as map dimensions) probably with a scale factor as your image is huge
then the image dimensions fill in automatically.
Not sure that the IMG is suitable for terrain it covers a huge area and although the model size is approaching square, the preview is very rectangular.
The altitude range is also very small in the model details
Import projection could have and effect Qgis usually use earth based grid projections
Paul