Making pictures appear without attatching them to a post? Mwuahahahah...now that is an arcane and deadly secret...the solution is borne of nude brain, toad eye, eel's hair, and monkey's tail...they must be stirred together in a pot kept under a slow boil for six yea- oh...I see that Windwaker has already told you...

*grumbles*

Anyway, yeah, you'll need to use a good image host and be sure to put the URL of the image in between two image tags. Additional help can be found here.