First, are you using png format? Jpg would definitely cause a problem.
Other than that it is pretty straightforward. Your Sprite should be the same size as the gimp file. If you resize in GIMP make sure interpolation is set to none. You can also try changing the layout scale if needing to zoom in or out.
There is nothing tricky here so if you can't fix it you will have to provide step by step what you are doing, a capx, or screenshot.