Loading pictures from user device at runtime

Hello, I am working on a project where I want the user to be able to use images from his own gallery (like pictures taken from the camera on android or iOS) and use this as the background for a 2d game. But I am completely clueless about how to do it and I couldn't find any info by googling around. I am quite new to unity so any help is really appreciated. I use C# in my projects.