1 - Single click on the image to open it up in a new window. (If you use the image directly from the blog post you will lose a lot of resolution.)
2 - Right-click (or ctrl-click) on the image, and chose the option that says, "Set as Desktop Background" or "Use as Desktop Picture". The wording may vary.
3 - If the image does not fit your desktop neatly, you may have to adjust the image (Mac: System Preferences - Desktop and Screen Saver - Desktop; Windows: Control Panel - Display - Desktop) and choose "Fill screen" as the display mode of your background image.
Any issues or suggestions please let me know. These calendars are an experiment in-progress.