Personally I keep copies of all of my customer's blog themes so that if they run into issues their images are safe but I know alot of custom graphic designers do not. Soooooo, if you love your blog theme and don't have a backup, here's how to save them....
Begin on your own blog and right click on the image you want to save,
Depending upon which browser you're using (Internet Explorer, Firefox, Chrome, Safari) a small pop-up menu will open up listing a number of choices.
I've highlighted the pop-up box in pink
Scroll down and click on the option that refers to viewing the image, this will bring up the image in a window all by itself such as in the example below...Next scroll over the image until you see a magnifying glass and the click ~ this will enlarge it to it's fullest version.
From there, select to save the image in an easy to remember location on your computer.That's it!
Remember that each browser can behave a bit differently and offer different options so if you don't see the exact choices, try to one that best relates to the situation and what you're trying to accomplish.
I can tell you that I originally tried this in Chrome but found that it didn't offer a "view image" option so I ended up using Firefox. Using multiple browsers isn't a bad idea either; it offers options for web browsing, so don't be afraid to try something new.