The images below our names are "Gangtags" and you can earn them various ways. Just gotta keep a look out. Right now,
Willsun is holding Gang Contests where only members of that gangtag can participate or people that want that gangtag can earn it. Applejack's Aces are the next in line for his contests and the remaining gangs are

(Rainbow Dash Gangtag) and

(Fluttershy Gangtag). The gangs don't really mean anything, they're mostly just to express who your favorite pony is. (Night Mares are the best though. FYI)

Only an
Admin or a
Moderator can grant or remove a gangtag.
For reference on how those run, here's the most recent gang contest that just ended:
Semper Pie, Party or Die (Pinkie Pie Gangtag)
This post over in
Dear Princess Celestia (Member to Mod and vice versa communications subforum)
Linkexplains how to earn them and some of ones that are available! The missing ones in that thread are relatively new:

Apple Artisans (Applebloom Gangtag)

Princess of Soul (Sweetie Belle Gangtag)

Scootaloo's Pro Scooters

Special Flowers (Cheerilee Gangtag)

Celestia's Champions

Night Mares (Princess Luna Gangtag)

Honorary Wonderbolt (Yellowstar Gangtag) (This is Yellowstar for reference

)
Then there's a couple tags which only one person has.
The other tags at the bottom in peoples sigs are earned tags, typically, through paintubes and such. Paintubes are where someone sets up a admin-sanctioned synchtube of something horrible that the apprentice must endure the full length of. The tags that say, "
I survived Newborn Cuties" were from the Paintube that involved watching 100% of the horrible
My Little Pony: Newborn Cuties movie.
"I survived Burnycon 2012" is for those that went to bronycon 2012, where a light fixture caught fire and everyone had to evacuate. "
SIX HUNDRED DOLLAR BRONIES" is from watching an auction where a brony bought some trivial and small item for a ridiculous amount.
Feel free to ask if you have any other questions!
