How do I change the wallpaper of the home screen that's behind the picture that I picked?

Just-AWESOME-old-me

Commendable
Jun 2, 2016
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Okay, so I have this picture that I want to have as my home screen (this is on a Chromebook), and it's a rectangle with a drawing and a white background. Now, this drawing doesn't look good Center Chopped and Stretched, however it does look good just Centered. HOWEVER, when it puts the picture in the background, it has a color that's underneath it to make up the empty space . . . AND THE COLOR THAT THIS CHROMEBOOK PICKED IS BLACK! Black, really? So this picture that has a white background is being shown on my wallpaper as a white rectangle in a black abyss. How in the world do I change this to a white background???