How can an imported graphic be deleted?
Here is how to delete a group:

To select the whole graphic and move it/delete it etc you use the group cmd to highlight all the mini-rectangles it makes, then select your tool (move,delete etc.) and then CTRL+right-click to execute that tool on the whole group.

Here is an additional tip on working with bitmaps in Eagle:

The import-bmp.ulp automatically puts your graphic at the origin. So before I run it I generally move my entire board away from the origin so that I can cleanly select the graphic group after importing it.

If you end up moving it in the middle of your board and then decide you don't like it, you can hide all layers except for the layer you put the graphic on and then the group command won't grab a bunch of stuff you don't want it to.