The best way to download on mobile is once you find your photo, tap on the first the picture and click the heart icon to save it as a favorite. You can then swipe right and select the next one. When you have selected all your favorites (if you have multiple), hit the X on the top right on the page. Then click the 3 Line menu on the top left of the gallery page, select Download Photos, check the box next to My Favorites, enter your email address and then click the download button. A link to the download will be emailed to you immediately. If you have just one photo, there is a cloud icon that will show at the bottom of the picture you can click to just download the one photo. Here are some helpful screenshots: