The excellent poster MrTwist has posted this excellent guide to posting images on this forum. However, he only explains one method. I'd like to explain another method, that works on a larger sample of online forums.
To post a picture on this forum, you first need to host that picture online. This means uploading it to an approved third-party website. The most popular ones include:
Tinypic, Imgur and Photobucket.
Tinypic and Imgur are easy to use, just follow the on-screen instructions. I don't use photobucket myself so I can't really explain how that one works.
After uploading, Tinypic gives you screen like this:
Look for where it says "IMG code for forums & message boards" and copy the line below that, then paste it in your post.
For imgur, you'll get a result similar to this:
Look for where it says "BBCode (message boards & forums)" and copy the line below that, then paste it in your post.
When you do it correctly, you'll see something like this:
The IMG tag in square brackets means that the forum will switch to image mode.
The blahblah.com/blahblah.jpg bit is the actual internet location of the image.
The /IMG tag in square brackets means that the forum switches out of image mode.
If you did it right, when you post the post, the forum will insert the actual image, like so: