First, the image needs to be hosted somewhere. Personally, I upload stuff to
ImageShack, but there are other places. Please, please, please do not leach images from other sites. It takes up their bandwidth (which costs money) without their permission.
Once the image is on the internet, you need to put it into your message. You will need the url of the image, then you put it in your message like so:
Code:
[img ]http://www.the-image's-url-goes-here[/img ]
(ignore the spacing inside the brackets, that's to get it to show up properly)
You can also click the little icon above the text field that looks like a picture of a mountain. That brings up a new window in which to paste the image url and puts in the IMG tags automatically. Or you can paste the url into your text, highlight it, and click the icon to put tags around the url.
If you use ImageShack, you get some code already done up for you. You use "Thumbnail for forums" to put a small image into your forum post that can be clicked for a full size image. Useful for giant images. There's also code that says "Hotlink for forums." This puts your image directly into your post. Useful for images under 600 pixels wide.