Still, confusingly, coming with different names in the Store and on the device(!), Image Resizer UWP is a handy little utility that just got itself an update, with new dark theme, an option to force the retention of aspect ratio, plus the option to open the newly resized image in Photos to admire/check it.
The dark theme is forced, by the way, and not just my test phone here picking up a system theme. But dark makes a ton of sense on AMOLED-screens, and looks classier on any phone, in my opinion. Some screens of the new version in action:
The new dark theme. Note that although I've only picked one source image here (to show the last check-box active), you can pick as many images as you like as source and they'll all be manipulated in the same way - they get listed by filename in the dialog above.