They look like this, in a few different colors:

You can find them in the forums and some other places, like the main page of the site. They can show up pretty much anywhere, so keep an eye out :3
But remember - if you click on an egg and it looks like you've been logged out of Gaia, DO NOT log back in. Close the window, open a new one, and type Gaia's url in yourself. Some people are putting eggs in their signatures/posts that link you to scam sites, so be careful. (It's always a good idea to check where the egg is linking before actually clicking on it. The legitimate url starts with "www.gaiaonline.com/event/easter2009/grabegg".)