Photoshop or Paint.NET is the inclusion and manipulation of layers. Layers exist within the photo editing software and give you the ability to transform anything under that layer. Imagine a stack of transparent slides, one on top of the other. If you want to draw a funny mustache on the picture, you actually start drawing on the transparent slide over top of the picture. It's essentially the same process in the higher end photo editing programs. Your background layer is you base and you apply different effects over top of that.
Layers allow you to make one set of edits that are independent of other layers. If you put text on one layer you can then move that text around independently of the other layers. If you have more complex effect like light or shadow set up you can move, delete or intensify the effect independent of other effects on the photo.
It will take a while to get used to editing with layers, but it adds quite a bit of power and functionality to the photo editing experience. Spend some time playing with layers (or watch the video below) to get an idea of how layers work. Get familiar with switching between layers.
Note, that when you save a photo in a layered photo editing software that it will use it's own format so that each layer can be preserved. When you save the image as a JPEG format (which you'll need to do to bring the image into PowerPoint), all the layers will be collapsed and you won't be able to undo anything that you've previously done. So, never save over your original image, but rather create an edit-file so you can always start from scratch. Don't save your edited photo as a JPEG until you're done using the editing software.
What tips do you have for editing photos with layers?