Documentation:WordPress Basics/Basics 1 Guide/Customize Headers

Some themes enable you to add customizable headers for greater personalization of your webspace. Remember that all themes are different; some might not even have headers.

• From the Dashboard, under the Appearance drop-down menu, click Header
• Click on Choose File, select an image (jpg, gif, png) from your computer and click Upload
• Adjust the slide ruler to crop the header image
• Click Crop and Publish
• Click Visit Your Site on the yellow notification at the top of your screen

Not all themes have custom Headers or even any header at all.