When you need a solid color source or background for your composite,
The Background node is just that,
It provides either a flat color plate or 4 types of gradients (color ramps).
Set the plate fill type in the Color tab of the node’s settings, and the format (size) in the Image tab.
When importing transparent graphic files, depending on the way the file was saved, you may encounter rough white edges at the transparency border.
In this case the image colors must be multiplied with the Alpha channel values.
In the Import tab of the Loader node,
Check Post Multiply by Alpha.
You can use a Matte > Matte Control node to further refine the edges.