What are the t.gif files on Preview, Export, Publish?
This is an invisible 1 pixel (1 dot) image used to achieve exact spacing of page elements in some cases. This image is not heavily used if Browser Compatibility is set to version 4 or later. In any case, this image is required for correct formatting of the page. The t.gif image is extremely small and takes an insignificant time to load. No matter how many times it is used on a page, the browser needs only load it once. So do not think this image causes any performance issue at all.