Background image trouble.

Hi,
I am new to wordpress. I am using the Singl theme and am trying to change the background image. I can get an image there, but only a single image. Even if I tile it, it still shows on the website as a single large image. I even created a collage in fotor and saved it as a JPEG, but it still only displays one of the pics used to make the whole collage image. In fact, I see the borders of the other images in the collage to the right and left, but I want it to display the whole collage, not just a magnified version of one of them. Could it be the size of the images I am using?

Thanks. BTW, I am code illiterate…trying to learn.