Set One Photo Viewer as default photo viewer
1. Open Windows start menu and click the settings icon (the gear).
2. Click “Apps” then “Default apps” and click on “Photo viewer” selection.
3. If you already have One Photo Viewer install then select it and your done. Else choose “Look for an app in the Store” and find One Photo Viewer there.
Reset settings file
If the software does not behave as it should, for some reason. You can reset the settings to default values by pressing SHIFT during start of the software. Best to start the software from Windows start menu. If holding shift while double click on an image you may accidentally open a whole bunch of images.
Make color transparent
Awesome!! Since you find your way here you have discovered an hidden feature in One Photo Viewer. Pressing Ctrl+Alt while click and drag left mouse over the image will make the color beneath transparent. Select suitable background (F6,F7,F8) for best view of the result. This is not so useful for pictures, since only the specific color beneath the mouse will become transparent. But can be useful for editing screenshots and other computer generated images. If the color beneath becomes black, then first click Ctrl+C then Ctrl+V (copy/paste) to make image format transparency compatible.