When you've completed your design in Mockplus RP, you can export your artboards as PNG images for better sharing and backup. Right-click on the artboard and select "Export artboard".
You can also choose an image size: 0.5x、1x、2x and 3x. For example, suppose you have an artboard that's 200 x 200px. Select "2x" and you'll get an image that's twice the original size - 400 x 400px.