It's very simple and easy to change the background image to your yahoo mail account. Lot of interesting and classic default themes available in yahoo mail themes.
Please follow step wise
1 Open Your Yahoo Mail Account - www.yahoo.com
2 Click on Right Side Settings / Gear Icon Button and choose themes option
3 Select any theme as you want to display on your background of yahoo mail and you can upload your own image from computer through click on My Photos Button
4 Click Done Button. That's all.
See Screenshots for More Clarification:
Choose theme as you like and click on done button… Enjoy !!
For any issue or support you can discuss with us: