How to put a background image?

[ Moderator note: moved to Fixing WordPress. ]

Hello!
I have the theme “Mountain Creek” on my website. It does not allow me to change the background color or image. I have tried two plugins (unsuccessfully) : “Full Background Image Manager WordPress” and “Full Screen Background Images”. They both did not work. I also tried with CSS without any plugin, but it did not work as well. What should I do? What am I doing wrong?
Thanks for your help!
Hugo