I have to admit that 99% of the time that I do this is to pull a prank on a friend, usually by editing something they posted on Facebook. It's fast and easy to do once you know where and how to use the tool. Much to the chagrin of my Facebook friend (and Austin rocker), Adrian Conner, I'm going to use her as an example of how to edit a picture in PowerPoint (MS Word also has similar tools.)
If this is all you want to do, you can stop here, and right click on the image and "Save as Picture." However, if you want to have some fun, then follow the next steps.
Click anywhere on the white portion of the background of the remaining picture, and it will become transparent.
Paste the picture in the same slide as the one you edited, and right click on the image and click on "Send to Back." This will put your edited photo in front of the image you now want in the background. Align your picture the way you want it by making it smaller/larger so that it begins to look like it is part of the new background. If you're like me, it make take a few tries and different backgrounds to get the effect you want.
It takes me about five minutes to do all the editing. It takes longer to find a good background to use than it does to actually do the editing. I find it to be very easy, and a lot of fun. Since many of us don't have editing tools like Photoshop, this is an inexpensive way to have some fun editing pictures.