Also ATTACHing an image file will work, but it consumes forum server bandwith, where links to external images do not.
Many free webhosts will not allow "remote linking", so images stored there do not display in forums (often showing a "Image hosted at ......." banner instead)
Boomspeed
http://www.boomspeed.com/
Do seem to allow remote links - and while 1 Mb for free ain't exactly generous, including large images in forum posts is a bit unfriendly to dialup users.
Oops!
"Use of Boomspeed for linking to anything other than auction images is restricted to Charter accounts only. Only charter accounts may be used for linking images to chat boards, websites, hosting of web pages (other than non- commercial support pages), etc. eBay ME pages, eBay chat boards, eBay message boards and eBay forums (this has been clarified to eliminate any misunderstandings, on 11/3/02) are considered auction usage and such linking is allowed with FREE accounts. Free accounts may also be used to host: Help, Support, Docs and Educational pages which do not consume a reasonable amount of resources. "
Maybe the forum users have PAID (charter) accounts, though I hardly think everyone sticks by that!
JPEG works well for photo or similar images
GIF or PNG works better high detail/block colour