Google removed the ability to display a custom background image on your Google Search homepage. However, there's a Chrome extension that can help you regain your little slice of customization.
Aside from allowing you to change the background image, the extension will let you do some other neat things, too. You can remove the Google logo, search and lucky buttons, footer, and promotional links.
Head to the Google Chrome Web Store and install the Custom Google Background extension.
After installing the extension, you can select where you'd like to use an image from. You can also randomize images if you provide more than one (from your local machine, a URL, or via search).
This Custom Google Background extension is a nice alternative to a theme, since it doesn't take over the entire browser window.