There is no way to add a custom emoji as they are part of an international standard that cannot be added to or changed without going through the international body that maintains the standards for them.
Newmoticons has great characterbased faces that you can add to your keyboard shortcuts. Also consider adding Kana and Romaji (Japanese) keyboards, they too have ASCII emoticons.
This. If the keyboard uses the proper unicode character set for Emoji, then it will look the same across all Apple devices, and look like it should on Android, etc.
If it's just inserting images, then that should work. But I don't believe they would show up inline with text. That GIF keyboard is kinda fun to play with.