In addition to a computer, you can do it on any phone. Go to the cert, select View Images below the TrueView pic. That opens their viewer widget. In the top right corner is the X to close, and the other button is download. It will open the image right in your browser. You can tap, hold, and Save to Photos.
To get below the forum 300k limit, I use the free Image Size app set to 800 pixels.