If it's already online, right-click it and select "Copy Image Location" (or similar option, depending on your browser). That'll give you a url that you can then post here surrounded by [img ] and [/img ] (without the spaces). I usually wrap all of that in a spoiler tag as well, just to keep threads neat.
If it's not online already and just on your computer, it's a bit more complicated. You'll need to upload it somewhere online - I usually use imgur, since it's pretty quick and easy.
