If you spend as much time with a Gmail window open as I do (thanks, Multiple Inbox feature!), you're probably over the rather dull default theme. And even if you're using one of the many custom themes available, maybe you'd prefer something else. Good news, everyone! You can use a photo from the web or your own computer as the background image, just like on your computer desktop.

Go to your Gmail account in Chrome or any other desktop browser. Click the gear icon in the top-right corner to open the drop-down menu, then click the "Themes" option.

An in-tab window shows you a bunch of backgrounds and color combinations you can use with Gmail on the web. If you like some of the ones you're shown, great! Click on any of them to apply them automatically. If you want more options, click the "My Photos" button in the bottom-right of the window.

You're given a view of a bunch of photos from all over your Google world: stuff you have in Google Drive, things you've sent to other people in Hangouts, and do on. You can select any of these, or upload a photo or paste a URL image with the options on the tabs above. For the sake of example, I'll click the "Upload a Photo" link to upload a wallpaper image I made for myself in Photoshop.

Ideally you want this image to be as large as (or larger than) the resolution on the computer you usually use to access Gmail, but the interface will accept images of any size. Click "select a photo from your computer" and then find the file you want to use.

After uploading the image, you'll have your very own custom Gmail theme.

You can repeat this process as often as you like, using most common image formats.