use the "img" tag like you do for linked images.
Go to user CP and bring up your attachments.
Either right click on it (with Mozilla/Firefox) and select "Copy Link Location", or left click to bring up your attachment and copy the url.
In your target post, use the img and /img tags around the url you paste in and it will show up just like a normal linked picture.
Edit: You beat me Christian, but I was more long winded so I win... :p