Thanks for reaching out to us! You need to crop your image to the correct dimensions. You can do this by opening Crop and selecting 5×7 from the Constraints dropdown. Note that the crop rectangle will start taller than it is wide and you may need to drag it so that it switches to being wider than it is tall (this will automatically happen by dragging around). Then you can crop it to the exact printing size.
Please let us know if you have any questions!
Regards – The Ribbet Team