By popular demand
Step one is to put your picture (jpg/gif) online somewhere world reachable.
No one can access pictures that live on your PC only. You have several options if you need webspace to host a file. Photobucket is a well known one, there are quite a bunch of others. Be sure to make 'm password free though!
Any webspace will do but some free hosting sites (geocities, angelfire) won't allow 'remote linking'. They only will show a picture when it's loaded from a webpage that sits in the same webspace.
If you have an internet account with a decent ISP, changes are they have some webspace for you reserved.
Once you have uploaded your picture and verified that it's visable for the world, you then can link them into your post, using the Img buttons. Include the whole url:
[ img]http://www.blablabla.com/fnord.jpg[ /img]
(Copy & paste the url into your post, highlight it and hit the Img button)
If you only want to link to the picture instead of showing it, use the Url button instead.
A couple of handy pointers:
Make sure the pictures are not too big. If they get too big, they'll mess up the layout of the forum (you get these scrollbars everywhere), and some people are still using dialup, so it will take forever for them to load a forum page.
Also, be cautious of linking to other peoples images without permission. They may not care about the picture, but may care about the extra bandwidth usage it generates for them. Or they do care about the picture and decide to replace it with something nasty....
Hope this helps