The easiest way is (as long as you know you have permission to use an image) is to view it in a webpage and right click on it, which then allows you to save the image to your computer. Then you can upload it using the Image button on the toolbar and place it in your page.

If you know a little bit about code you can also do it another way. You need to put an image in your page (think of it as a dummy image), then double click on it and click the Code button at the top of the popup. You then need to paste in the image URL over the existing reference (you can get this from any image on the web by right clicking on it and selecting Copy Image Location), and you will also need to change the word 'width' to 'size' in the code.