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: