Find a picture of want you want and right click it to save it to your computer. If you don't have an account with a image hosting service (photobucket, imageshack and tinypic and three commonly used ones), then make an account at one (the three I mentioned have free image hosting. Upload the images to your account and use the IMG codes to post the picture here.
The code will look like this (without the asterisks)
[*img]image url[*/url]. Say, for example, I want to post one of my pictures.
[*IMG]http://i53.photobucket.com/albums/g54/caffeinatedjoy/Miscellaneous/Backshot.jpg[*/IMG]
Take out the asterisks, and you get:

Just make sure your image is no larger than 600 x 600. If it is and you can't or don't want to resize them, then just post the link, without the img codes wrapped around them.