EAGLE: Placed bitmap on tStop
Hide all the other layers (including the origins layer) by selecting the displayed layers window, clicking "None", and then clicking on the tStop layer number (29) as shown below: 



Once hidden you can then use the group tool to select all of the bitmap on the tStop layer:



Then select the delete tool, right click on the object, and choose delete the group:



Generally there won't be anything you've drawn on the tStop layer unless you've added it manually as they most stuff will be linked to the part itself and so won't get selected in the group as the origin for the part is hidden.



As a side note, I would highly recommend not using the import-bmp ULP in the layout. Instead what you should do is to create a new library to use for images, then create a new footprint for your image, and finally run the ULP in the library window.

In this way you can simply use the add component tool in the layout window to add your image footprint. This will allow you to easily move the image around just like any other part, and easily delete it. Plus if you mistakenly import it to the wrong layer in the library, you can simply group all and move it to the correct layer without having to worry about other parts.