Don't be fooled by the pretty images.
While painting/photos can express multiple, deep emotions, they are unique. The value added by a copy it's none.
Moreover emojis are for 99%+ simple nouns ("hand", "pig", "dog", "smile"). And for thing that are not nouns, you still need to understand that they are logograms, with a "fixed" meaning. Kanjis are not better than Romans - they are just different alphabets.
And if you think about it long enough you will find that there is already a 1-to-1 mapping behind the "emojis" and the words (by definition, the unicode is the mapping of numbers - the code point, to meanings - the name, and to images - the pictogram, so you can have U+1F436 associated to 'DOG FACE' and ) , so they are "informationally dense equals" between each others.