Windows 10 - when attempting to open multiple image files simultaneously, instead of opening they become my desktop background.

This has been going on for a couple months at least, but not much longer than that.

I have images set to open with paint.net (basically a ghetto version of photoshop, but free). Used to be if I select multiple images on the desktop or some folder and hit enter, each would open in its own tab in paint.net; but lately doing so is just setting them as a slideshow style desktop background. Desktop background is typically just a single image - slideshow is not enabled, but it turns itself on every time this image fuckery happens.

Any thoughts on how to get it back to normal?