Is there part of a photo you'd like to cut away? Do it with the cropping tool in PowerPoint. Then resize your picture back to the desired dimensions. To see how, click Play video, above. (In case you've muted sound on your computer, note: the video has narration.)
Note The version of the photo in your presentation is a copy of your original, so cropping it won't affect your original. Also, in the PowerPoint version, you can restore what you've cropped away.