How can I make a build use external sources?

Example: (this is not what I'm trying to do, but knowing how to do this will solve my problem)
1) I wanna have a folder named "background" in my game's directory. It should contain an image called "bkgr.jpg" or whatever. And I want the game's background to be this image. When the image is changed (replaced with another image called "bkgr.jpg") the game's background also to change.
2) I want the player to have the option to select an image from his computer (browse it), and select the background this way. Let's say that there could be a button.

 

Load texture from file in your level.

Monitor directory for changes using one of the functions provided by OS.

For example:
https://msdn.microsoft.com/en-us/library/system.io.filesystemwatcher(v=vs.110).aspx
^^^ Not sure if this one is available within unity.

Upon detecting the changes, reload texture.

Also see:
https://docs.unity3d.com/ScriptReference/Application-dataPath.html