Log in to your Tumblr dashboard, then click the drop-down arrow next to your primary blog name. Click the name of the blog to edit. Click the “Customize” tab to open the blog in Edit mode. The Customize panel appears in the left sidebar. Click the “Upload” button in the Header Image section of the Customize panel. The file selector opens. Browse to and click the animated GIF to upload. Click “Open” to upload the file. The progress indicator displays while the file is uploaded. When the file is finished uploading, the Clear button appears next to the Upload button. Click “Save,” then click “Exit” to close the Customize panel and return to the dashboard. Click the URL for the blog in the right sidebar to view the animated GIF header on the blog. When you upload an animated GIF image as a header graphic, the image fills the entire header section, no matter the size of the original image. Therefore, there is no need to resize the image to make it larger for the header.
