The first way is to simply link to a picture from another web site, such as Photobucket (there are others, but we'll use this as an example as it's all the same).
Step 1: Get the link url of the picture you want to link to from the web site. The URL (a.k.a. "link") should look something like this: "https://2img.net/h/i3.photobucket.com/albums/y88/webby4x4/4X4REV-Cruiser007.jpg"
Step 2: Simply past that URL into the post that you're making.
Step 3: Simply type "[ img ]" (without the quotes and without the extra spaces) BEFORE the image, and type "[ /img ]" (again, without the quotes and without the extra spaces). What you've typed should look like this...
[ img]
https://i.servimg.com/u/f66/14/66/73/91/ad_des10.jpg[ /img]