The first thing you aught to think about when you're copying a picture is who's picture is it?
Most places on the Internet are all about sharing, especially with people who are in schools trying to learn.
But sometimes web sites have pictures that are not to be shared.
There are trillions of pictures on the Internet so if you see a sign asking you not to copy this picture just go find another picture someplace else.
If it looks like the picture is okay to share it's really as easy as drag and drop. Here Try with this groundhog.

Just point your mouse so your cursor is on top of the picture. Now click but hold down the mouse.
Start dragging the picture to your desk top, if you're doing it right it should be shaded in blue.
Then just drop it on an empty space on the desktop.
If your browser is taking up all the screen just resize it to there is some of the desktop showing.
Now that it is on your desktop you can put it inside your presentation by draging and dropping.

It's important to give credit to the people who you got the picture from.
For example I got this groundhog picture from www.freeclipartpictures.com/