I forget say, this feature might have a way to disable it on some servers...

The idea is when write some words like -hello-, the word will replace with photo when send it at chat, and we can add other custom shortcuts and images.

This would be really nice to have, but all they are gonna say is to make your own chatbox so I wouldn't expect anything to happen.

If it's already possible to script it (no matter how difficult) it's not gonna be added to MTA as a feature since there are more important things for the devs to do.

I would never say never, but I won't expect this being added into MTA in the near future. Chat in MTA is there to allow players in a server to communicate quickly with each other, and while on a DeathMatch rampage, a hard car race or in a serious roleplay situation, I don't think that most players would care about seeing ":)" displayed with an image instead of text. In fact, it can be argued that it could be disturbing if for some reason the chat gets flooded with emojis. Of course, if emojis were a core part of the experience, like they can be in a dedicated chat application, it would be crazy to not pay attention to them, but MTA isn't close to that and for me it's something that wouldn't make much sense. I don't remember seeing any multiplayer game like Battlefield, LoL or such adding emojis to their chat systems either, because as in MTA the chat in those games is just a tool which does its job pretty fine by just displaying plain text. I see no point in devoloping emojis which would not add something significant to the gameplay, increase the size of the client files and even generate mixed opinions from players.

Like people said above, it's already possible to script a pretty good looking chatbox without that much of an effort by using CEF and some clever HTML code, if you think that you need emojis in your server's chatbox right now, although this is not the only way to implement this (and maybe the old school DirectX drawing functions are faster).