You are making great progress on this.
Here is a document from my website that lists the YafaRay features and where to find them (Including backgrounds)... YafaRay Supported features
As Micheus said, you can render with a transparent background and then add your render to an existing image in Gimp or other image editing program. The Image plane method should work but you could also build a flat plane and add a texture to that as well.
Edit:Transparent background (Alpha under YafaRay Render options/General options tab) is not working in the latest version of YafaRay. You will need to use YafaRay version 0.1.99.4
oort
Here is a document from my website that lists the YafaRay features and where to find them (Including backgrounds)... YafaRay Supported features
As Micheus said, you can render with a transparent background and then add your render to an existing image in Gimp or other image editing program. The Image plane method should work but you could also build a flat plane and add a texture to that as well.
Edit:Transparent background (Alpha under YafaRay Render options/General options tab) is not working in the latest version of YafaRay. You will need to use YafaRay version 0.1.99.4
oort