I hope this is what you're asking?
In the Admin Control Panel:
Go to Images>Upload an Image
Upload an egg sprite for your adoptable. It is necessary for the next step.
Go to Adoptable>Create New Adoptable
Enter something like this:
http://i62.tinypic.com/v5fpy1.png
(The part not pictured is alternate outcomes. If you have other images to use, you can use them here.)
If not, a good pixel art tutorial is here:
http://www.pixeljoint.com/forum/foru....asp?TID=11299