Ran into the same problems trying to figure out to insert a background image into a Generic Control. After researching and a bit of trial and error, I came up with this approach. The good thing about it, is that this now allows --
the user -- to input an image into a generic control and then set it so that it is a background image.
Adding a class style for the background image to our .css file...
In our .css file, our Controls are in a div tag called "Main" with the control itself is called "Body": Although you may find the redundancy of the styling not necessary, we decided to keep it.
With the Generic Content opened for editing,
--> I inserted an image using the icon in the upper tool bar (insert an image)...
--> I Right Clicked the image for properties and "
deleted" the height/width dimensions (setting the dimensions to zero hides the image)
--> I set the image's CSS Class properties to "backgroundImage" (add to the .css in the previous step)
--> alternately, I edited the HTML generic control and inserted -- class="backgroundImage" --
Upon clicking the preview, the image shows up as a background image with the text floating on top.
Now the user can adjust their web page so that it fits around the various image contents as desired.
Hope this helps others or at lest gives them something to think about. ;o)