Somewhat confusingly coming with different names in the Store and on the device(!), Image Resizer is a new UWP utility that does exactly 'what it says on the tin'. Such applications aren't new to Windows Phone, of course, but it's always interesting to note full Windows 10 apps that also work on tablet, desktop, Continuum, etc. too.

The interface is extremely barebones but Image Resizer UWP does work, as shown below:

There's rudimentary help, but mainly targetting Windows 10 desktop users....

A simple image picker is followed by this dialog UI - very basic but it did the job for me....

...and here's the proof, with my photo reduced to 720p, as desired.

You can grab this free UWP app in the Store here. It's slightly odd that simple resizing of images can't be done with Photos or Lumia Creative Studio, but at least there's a UWP app that does the job.