Yes, .gifs can be used when sending messages out of NetCommunity or Online Express.  This is a supported file type however the email client used by the end recipient may cause the image to display the first frame only instead of the full animation.  So far this only appears to affect emails viewed in Outlook, other clients such as Hotmail, Gmail and Yahoo appear to animate the image correctly when received.

See the below articles for more information on adding images to your Blackbaud application:
How do I upload images for Online Express emails?
How to insert an image into a NetCommunity email message

Blackbaud will update this article with more information once it is known.