You don't. Entourage doesn't support animated GIFs.
Not entirely true...
I have sent you an example off-list, where an animated gif will animate in
Entourage.
Entourage has two levels of HTML built in - 'simple' html includes things
like text size, font, bold, underline and embedded images (in 2004). These
messages can be generated by entourage and viewed in entourage.
Then, there is 'complex' html that includes things like tables, animated
gifs and other fancy mark up stuff. This cannot be created directly in
entourage - you have to have an additional HTML Editor to create the code
(Entourage is for email - if you want a web page, use a web page editor!).
Complex HTML is not viewed using the built in display engine, but when it is
encountered, it is passed off to the MSIE rendering engine (providing the
Entourage Prefs are set up for this, which they are by default). And, guess
what? MSIE can animate gifs, so it will animate any gifs encountered in
complex HTML in Entourage.
The trick is to generate the complex HTML!
In Office 2004 this is easy - create your email message in Word, embedding
any animated GIF you want to (although the animation won't necessarily
display in word), then select 'send to recipient as HTML' from Word's File
menu. This will create a complex HTML message in entourage, with GIFs that
will animate in other people's entourage as well. Prior to Office 2004 you
had to use a third party script 'Send Complex HTML X', available from
<
http://www.scriptbuilders.net/> and a web page editor.