Err, you just need to go to your ACP for the site, go to "edit existing adoptables" or something like that and change the egg image for the selected species. :3
If it's the egg image, it should be in existing adoptables... but I won't let you click the gear? Go to your images, copy the image url of the desired egg image, then go to your database. Go to the adoptable table and change the image from there.