How to Make GIFs Work on Tumblr Themesby Alan Sembera
You can often draw more attention to your Tumblr blog by using animated Graphics Interchange Format files as part of your theme. To get the animation on your sidebar or description area, you must first upload the GIF file and then add a link to the file in your theme's HTML code. If you have trouble getting the animation to work properly, you may need to modify the original file to meet Tumblr's image standards.
Add Image to Theme
Upload the animated GIF file to Tumblr from its static file upload page (link in Resources). Tumblr allows you to upload animated GIF files as large as 5MB from this page, as long as they are for use on your theme. Alternatively, upload the file to an image-hosting site such as TinyPic, PhotoBucket or ImageShack. For other sites, keep the file size under 10MB.
Copy the image URL provided by Tumblr or the image hosting site.
Go to your Tumblr dashboard and click "Customize" to access your blog theme's settings.
Click inside the Description field at the point where you want to insert the animated GIF file and enter the following HTML code: <img src="URL"> Replace "URL" with the URL for your animated GIF. Keep the quotation marks around the URL. The final code should look similar to: <img src="http://imagehost.com/imagefile.gif">
Click anywhere in the preview pane to see how the image looks. On some themes, you may need to add a width attribute to the code so that the image fits in the sidebar. To set a width of 300 pixels, for example, modify your code as follows: <img src="http://imagehost.com/imagefile.gif" width="300">
Click "Save" and then "Exit." If the image doesn't display properly, or the animation doesn't work, wait a few minutes for Tumblr's server's to update your page. If it still doesn’t work, you may have to modify the original GIF file and then upload it again.