Here's some general tips on using BBCode image tags in forums:
http://www.phpbb.com/phpBB/faq.php?mode=bbcode#9
I've been using photobucket.com (although officially they don't allow adult images).
For example, this picture host will give you a direct URL to a picture you've uploaded - http://i16.photobucket.com/albums/b29/matadorstuff/71.jpg
To display that on a forum, you just add the [ IMG ] and [ /IMG ] tags (leave out the spaces) around the URL...
Photobucket doesn't seem to have the option of "thumbnail" images which link to the full image, but other picture hosts do - you just copy and paste some code into the forum message body. That code, which they would give you anyway, would take a form like:
[ url=**URL of FULL image** ][ img ]**URL of thumbnail image here**[ /img ][ /url ]
(again, leave out the spaces between the parenthesis and the characters inside)
*BTW, the girl above is Brook - http://www.peachyforum.com/forums/127584/ShowPost.aspx