Many times you want to post an image on the forum. There are several ways of doing this. I will show the forums inbuilt functions first, then I will show what I do.
First, the image has to have a URI, for pictures on the web already, this is easy to get.
See this wikipedia page for the screenshot of Ubuntu 8.04: http://en.wikipedia.org/wiki/Ubuntu
Clicking on it gets you the image information page, and then click that image to get the image itself. For most web pages, you can right click an image and copy the URI, and then paste it (how depends on the browser)
Here is the URI of that screenshot: http://upload.wikimedia.org/wikipedi...buntu_8.04.png
To post an image with just code, you use the [img][/img] tags, with the URI of the image included.
In the GUI editor, the picture of the mountains and and sun (it is mostly yellow) will give you a dialog to automatically paste the code, just give it the URI of the image.
That will embed the image in the post. For larger images, you should use thumbnails (images that are small, but are linked to the larger version).
To attach one to a post, you use the paper clip icon and upload it from there.
My preferred way of posting images is to use: http://imageshack.us/
Without an account, you can upload images (<3 MB) and you get the code to paste the entire image or a thumbail (in a forum, blog, and website). You can also resize images and use some more options. With an account, you get more features.
Here is a picture I took in Maine uploaded onto Imageshack.us: