If you are not a Photoshop master, here is a crude way to cut out an object out of an image in PowerPoint:

1. Take an image
2. Draw a freeform shape over it
3. Remove the image
4. Fill the shape with the original image
5. Select crop, and then "fill", then scale up the cropped image until the shape is filled with just the bit that you intended
6. I added the original image back in the background