I only saw one other thread on the topic and it was locked after being largely unproductive.
This is not about the ability to right-click a single image and selecting to edit in paint.net. This is not about setting the default VIEWER (and editor) for images. This is not about setting the default apps in the standard Windoze UI widgets. All of those are quite simple. It's about altering the "root" file association for ONLY the image editor, leaving the default app for VIEWING to whatever you have in place now. Why would I want to do this?
Because it's the only way to make it so you can select MULTIPLE images and right-click and edit with paint.net.
The other thread seemed to indicate there was an install-time tick box to set the editor - it's been so long since I've done an install I don't know if that's the case or not (or if it alters only the editor and not the viewer as well) but at this time there does not appear to be an option in the settings UI. As this is a simple refedit I would suggest that as a possible feature enhancement.
There are "apps" to do this "in the wild" (the other thread conveniently alludes to the first hit on Google, a sponsored link - use at your own risk) however the required regedit is astoundingly simple. To my knowledge this works in every version of Windoze since XP......
Anyway - to ONLY change the default image editor:
Search keys only for SystemFileAssociations
Open that key and navigate to image
Open that key and expand down to shell\edit\command (if you want to be safe at this point right-click the command key and select "export" - save the file somewhere and if you want to reset the edit just double-click the file and run it)
The default value will look like this:
Change that value to:
"C:\Program Files\paint.net\PaintDotNet.exe" "%1"
...or whatever the full path is to the executable for your installation of paint.net (if in doubt you can even search the registry for an existing entry in the paint.net key)
The change is immediate.
Hopefully someone finds this useful.