For an image to receive the coveted status of becoming an official emoji, it needs to satisfy multiple requirements set forth by the Unicode Consortium, the nonprofit that reviews all emoji applications.
Among the factors taken into account are: expected usage level, image distinctiveness, and compatibility with apps that regularly use emoji, such as Snapchat and Twitter. But it's also important that the emoji have references beyond its most literal meaning. The example that Unicode offers applicants is a shark, which can be used as the large saltwater fish, or, more creatively, to describe a card shark or loan shark. (Whether or not Unicode predicted how users would interpret the eggplant — which is, at heart, just a vegetable — is up for debate.)
But beyond metaphorical meanings, there are also the emoji that we think mean one thing, but were actually intended for other uses. The peace sign? Nope, it isn't actually a peace sign. The angry face blowing steam? Nope, it isn't actually angry.