Under usercp, follow the bottom link on the left - "Attachments"
It'll list all the attachments you've uploaded. Find the one you want, right click the link to it in the left column, "copy link location", then paste it into your new post, surrounded by [img] and [/img]
“The egg hatched...” “...the egg hatched... and a hundred baby spiders came out...” (blade runner)
“Who are you?” “A friend. I'm here to prevent you from making a mistake.” “You have no idea what I'm doing here, friend.” “In specific terms, no, but I swore an oath to protect the world...” (continuum)
“It's a goal you won't understand until later. Your job is to make sure he doesn't achieve the goal.” (bsg)