Remember Microsoft's "tude" is that something like this that could be fixed by the user resizing or changing a setting to size to fit is not considered a bug. Yes it does annoy folk but we are talking Windows here.
Everything was working swell until (I am pretty sure) Microsoft gifted me with "Win 10, version 1703, Successfully installed on 9/30/2017".
Now all the (jpg) Background Pictures (and only the Background Pictures) have expanded so they overflow the screen. Everything else (icons, etc.) visually unchanged.