Not an eraser tool as in image editing program. But if you tell us which
version of PowerPoint you use and which effect you want to achieve, we might
be able to come up with a workaround.
There is no eraser tool but for simple jobs, you may get away with placing a
shape over an image. You might further group the image and the overlaid
shape, export the group as a picture and replace the existing image with the
newly exported picture.
To add to what everyone else has said, note that PowerPoint uses "Drawing"
tools, not "Painting" tools. With drawing tools, you have object that you
can move around on the screen and modify their properties. With painting
tools, you have no objects, just a bunch of pixels. That is why you can have
an eraser tool with painting because you are simply erasing some pixels
(just like taking an eraser to a piece of paper).
--
David M. Marcovitz
Author of _Powerful PowerPoint for Educators_ http://www.PowerfulPowerPoint.com/
Microsoft PowerPoint MVP
Associate Professor, Loyola University Maryland