There is no such option, since Outlook 2007 doesn't support animated gif playback. What you'll need to do is is 'save' the attachment to 'My Documents' or somewhere, then double-click it. It should then open and play in it's associated application (on my PC that's MS Photo Editor, though I have not tested it with animated graphics, so I'm not claiming it can play them). If that doesn't play them, right-click the gif file you saved and choose "Open with...", then point it to any web-browser (you may have to use "Choose program" at the bottom of the"Open with" menu.
Once you've selected a compatible program to play them, you can enable the option to "Always use this application for this type of file", by ticking the box at the bottom. That will 'set' the association so in future you only need to double-click the saved gif file for that application to launch and play it.
Edited by pip22, 22 March 2008 - 09:13 AM.