Well first of all those are called "plz accounts." they are accounts just like a normal account only they are dedicated to animated icons and emoticons. liiike this one
ooor this one
this is how you do it:
1. :icon imhappyplz: without the space of course.
2. that makes
3. Make sure the account you just typed exists.
4. if there is an icon that you want to know what the plz account name is, just hover over the icon. It will tell you. Then just put "icon" in front of the name in the format I showed you and there you go~
5. There are some accounts that collect these plz accounts but i cant find any right now...you'll have to just find them by surfing DA. but a friend of mine has quite a collection
[link] check it out.