Using a repeating image for your Tumblr background often makes your blog more interesting and eye-catching. If your theme doesn't have a tiling option, you can make your image repeat in one of two ways. The easiest method is to add a single line of code to your blog's custom CSS field. This entry overrides your theme and creates a tiled background. You can also edit your theme directly to make the change. In either case, you must start with a background image small enough for tiling -- Tumblr won't automatically reduce large images for tiling.
The Easy Fix
Sign in to Tumblr and click "Customize" on your dashboard to access your theme settings.
Click "Advanced Options" at the bottom of the Customize pane, and enter the following line of code to the Add Custom CSS field:
Click "Save," click the back arrow, and then click "Exit" to return to your dashboard.
The Harder Method
Click "Customize" on your dashboard, and then click "Edit HTML" to open your theme's HTML code. This code controls the layout and appearance of your blog.
Go to the beginning of the style sheet section of code. You can find the start of the style sheet by pressing "Ctrl-F" and searching for "