I think the title is explicit enough, I've always looked for an IRC client where I could put some custom emoticons.
Here's how I think it'd work :
-You import an image and assign it to a text (Could be :) or plain text like on twitch, FrankerZ for instance)
-Those who don't use Quessel would see plain text, but if you or someone types the assigned text, the emote show up.
This description looks quite obvious but anyway, hope you get the idea ;)