First you're going to need a little webspace - a place to upload pictures.
f'rinstance, my domain is invisibleband.com
I take a picture i want everybody to see, save it as a file on my hard drive. for the sake of this post, we'll save it as "test.jpg" (no quotes).
To get "test.jpg" from my hard drive to the web, where all can access it, i'll need to use an FTP program (File transfer protocol, but you don't need to know that) such as cute ftp or WS-FTP (that's the one i use). When I got my domain name/webspace, my provider gave me his ftp site's address and a password.
I open my FTP program and drag "test.jpg" into my webspace folder, thus uploading it from my computer to my webspace's server. It can now be accessed by any web browser in the world at http://invisibleband.com/test.jpg
(this is all taking place in your imagination - this link doesn't work).
To access this image within a jemsite forum post, i will need to put a tag that says "[img]" before the "http://" part and a "[/img]" at the end of the address. (NO quotes!)
The forum will put the "img" tags in for you if you select the whole address and click the "Img" button under the "Subject" line.
In this particular software, if you don't use an "img" tag, the address will, by default, turn into a regular hyperlink which users can click on. Some prefer that, as they don't have to wait for the images to load each time they view the thread.
If you see a page wtih an image on it, click on the 'Quote' button in their post and their code will come up and you can see a real-time example.
hope this helps. if you need to contact me on AIM, feel free.