For the PC: Right click with your mouse on the image you want. From the dialog box, select "Save Image As..." Save the image with a file name that makes sense (like "clouds") so you can find it in your computer easily.

For a Mac: Click on the picture you want and hold down the mouse button. Then select "Save Image As...". Save the image with a file name that makes sense (like "clouds") so you can find it in your computer easily.