There are a couple of possibilities,they could have used
this code adding html to it for the images, or they may have used the custom profile option and added images to that.There's a tut on the custom profile field somewhere,not sure where though.It discusses adding image codes to a custom profile field and having it show in the posts. Just do a search for it.
Your third option is to ask the owner of the forum you seen it on...I'm sure they will have no problem telling you which code they used. Good Luck.
If I find that tut...I'll post it for you.
Custom profile field options tut can be found
HEREJust found it