Click on the name of your blog in the Tumblr dashboard and select “Customize Appearance.” Choose “Edit HTML” to open the source code for your current Tumblr theme. Press “Ctrl-F” to open the search feature of your Internet browser and type “background:” (without quotes) in the search box. Press “Enter” to locate the code that adds the background image to your Tumblr page. Add the “background-size” CSS3 property to the code, after the line that specifies the location of the background image. You can use the property to specify the width and height of the picture. For example, if you want the width of your background image to be 500 pixels and its height to be 800 pixels, you would add the following line: background-size:500px 800px; Click “Save” to save your preferences and resize your Tumblr background. Writer Bio
