What are displayed on Gmail's interface may not be what the person on the receiving end sees (unless they're using Gmail of course).
Gmail simply replaces a piece of text, such as ; ) (no space) with an emoticon 😉, in much the same way as this forum does.
Some email clients do allow you to replace textual emoticons with images, but only for display purposes on your end, and the icons will differ from the ones available in Gmail.
I suppose it would be possible to copy all the emoticon images that Gmail has and then insert them into emails if you write them as HTML, but i've never had any use for HTML emails so haven't attempted it myself.