Windows 10 "Always use this app" checkbox missing

Solution 1:

As of August 2019, the latest version of Windows 10 sets the default application for viewing images in the "Apps" settings.

1. In the search bar, type "Settings".
2. Select "Apps" in settings.
3. In the menu at the left, select "Default apps".
4. Choose your default photo viewer app.

Solution 2:

There is different look (and behaviour) of that dialog for any file type:

Context -> Open With ► -> Choose Default Program…:

or Context -> Properties (General tab) -> Change… button
or (the same) Change program… button in Control Panel\ … \Set Associations:

Conclusion: The last dialog is intended to set default program for all files of given extension; therefore, the Always use this app to open … checkbox would be superfluous here.

Please note:

• Context key is used in above text as an abbreviation for mouse right-clicking i.e. usual request for a context menu which is performed on the keyboard either by clicking the Context key (menu-like looking key usually to the right of the space bar between RightAlt and RightCtrl) or by Shift+F10;
• images were taken from Windows 8.1 but behaviour keeps unchanged and should apply to Windows 10 as well;
• (too long to comment; I'll meet your wish if you do call for deleting this answer.)