How to stop MS Paint from being the default photo viewer on windows 10

1. Sign in to your computer using an administrator account and open the Windows Registry Editor. In Windows 8, click "Search" on the Charms bar, type "regedit" in the search field and press "Enter." In Windows 7 and earlier, click the Start button, type "regedit" in the search field and press "Enter."

2. Navigate to the "HKEY_CLASSES_ROOT\SystemFileAssociations\image\shell\edit\command" key in the left pane. If you are at the correct location, the "Data" column in the right pane should display: "%systemroot%\system32\mspaint.exe" "%1"



3. Right-click the "(Default)" entry in the right pane and select "Modify."

4. Replace "%systemroot%\system32\mspaint.exe" in the "Value Data" field with the full path and filename of your preferred image editor's excecutable file. If you are changing your default image editor to GIMP 2.8, for example, the text in the "Value Data" field should look like this including the quotation marks: "C:\Program Files\GIMP 2\bin\gimp-2.8.exe" "%1"
in my case, it is changed to "C:\Program Files (x86)\FastStone Image Viewer\FSViewer.exe" "%1"

5. Click "OK" to save the new setting and then close the Registry Editor. The change takes effect immediately.