Android：使用图库中的图像更改程序背景

So I want to have the option for the user to change programs background with his own picture from gallery. I have a button and I saved the images as bitmaps and tried to use button.setBackgroundDrawable() to do so but it puts the picture over the button so that only the button text can be seen and it also doesn't make the picture fullscreen. Here's some of my code:So I want to have the option for the user to ch