No I really have no idea. I looked for a guide here on the forums but didn't find one. Or I am just blind. Either way computers and the internet are not by any stretch my forte at this point in my life.
Before what Aregon says, you have to get it online in the first place. The best method is through an image hosting site like imgur.
Afterward you'll be able to click the photo icon on the toolbar and then copy the link that the hosting site gives you.
Occasionally you'll encounter a format that the forums don't support, but most sites give you multiple methods. You should be able to see the image before you post it - else a small broken picture icon will come up.