Using Eraser Tool on 'placed' Image (png)

I have been on every forum trying to find out how to do this an d cannot get a solid answer. I placed in image on top of another image in my artboard. Now I am trying to erase only part of the top image to show some of the background image. The Eraser just denies me use of it when trying to erase part of the image. Do I need to convert it to something else first? Thanks for any help on this.