I've added a couple of resources in the Resources folder in this section. 

They explain how to find copyright and how to use Creative Commons.

Please read them.

Once you have found an image and checked it for copyright you need to download it or save as image by right clicking it.

Many sites have  copyright information across the image. It's very uncool to use one of those images. 

An organised person would have 2 folders: Originals and Resized. Many of us don't but it is good practice to do so.

You'll find the suggested sizes in the Resources folder files or in the For Staff section in Moodle.