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]
Imagine a pathologically idealistic, anti-religious, culturally-relativistic, iconoclastic, socially-irreverent, sociologically-progressive signature line here. Or don't.