Don't bother, it's just not possible. Outlook 2007 will use the rendering engine of Word. As it is now, Word 2007 (which will be installed when using Outlook 2007) does not support animated gifs. Only the first frame of the animated image will be displayed -- so better make this as informative as possible!
Strange how previous versions of Outlook had this functionality, and the "new" version doesn't. Apparently, the change from rendering engine, from Internet Explorer to Word, was done for "consistency" purposes. They did accomplish that -- it's now consistently harder to get anything to work in Outlook.