I see no mention of a shortcut to save images in the list of shortcuts published here:

• https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly?redirectlocale=en-US&redirectslug=Keyboard+shortcuts

How do you select the image without using the mouse? If you have to use the mouse to do that, why not just use the right-click?

If you are viewing just the image, you can save it using Ctrl+s.

But if the image is in the page, as finitarry asks, how would Firefox know which one you wanted to save?

As you probably know, you can alter Firefox's behavior using add-ons, and there may well be an add-on that has this feature. I've seen add-ons for saving images with a single mouse click, including this one: Image Toolbar. Perhaps if you search around you can find one that works with just the keyboard.

If there is no add-on, you might be able to create a Greasemonkey userscript to do it. Edit: Actually, that might not work because saving to the file system involves privileges not normally given to websites...