I want to attach a dirt image to the ground plane. Maybe my question should be where are ground plane images or objects to attach to the ground plane?  I look on Daz and Renderosity and don't see any.

As to the question on how to do it, I gather I just use the material room?

material room , either use the ground image node thats there or create a new 2d image map node and connect it to the diffuse chanhhel. Just click on the top left of the image node and drag it over the diffuse node plug icon.

The ground plane is rather expansive.  An alternate way, use a 1-sided flat plane and match the image's resolution with the x- and y- scale for the prop.  Rotate the prop so it is parallel with the actual ground plane and use the Drop to Ground feature found in the taskbar.  IIRC, it might be under Edit.  Afterwards, you can scale that plane to fit your scene.  And it would be easier to use displacement mapping to get greater detail.

Also, if you do use the ground plane, don't forget to uncheck Shadow_Catch_Only on the PoserSurface node.