I have an iMac, and simply click on the picture to darken it, go to the tool bar above the desk top and click on Edit, then choose "copy" then in my case I choose to use the TextEdit which is sorta like a loooong post-it note, and then go again to Edit and click on "Paste".
I then can put it in any file I want, or give it a new name and put it anywhere. Doing this has let me discover that I love best the strong colors that dramatically show each pattern.