To post an image right in the thread, the code is simply the URL between opening [*img] and closing [*/img] image tags...without the asterisk. I had to put the asterisks in for the code to show as text here.
[*img]The URL of the Image[*/img]
To see it exactly, simply "edit" a post that has an image and look at the code. Don't worry, the computer will not let you accidentally change another person's post.
A good size for these forums is 700 - 800 pixels wide. Larger than that and the image stretches the whole page to accommodate it and we all end up having to scroll sideways.