Some forums allow pictures to be added as attachments. See the second box in the additional options block below.
I store my pics online at Smugmug. When I want to post a pic, I right click on the image in Smugmug to "copy the image URL", go back to the forum and select the "insert image" box (one of the selections available above the message box), and paste the image url in the insert image box.
I have used Smugmug for 5+years. They charge for the service, but it is worthwhile to me. Besides being able to post pics online, there are may other advantages to being a member (safe storage, printing service, sharing options, etc.). Smugmug offers a free trial. If you decide to join, you can get a discount using my coupon number - Smugmug coupon code = bQItjZEfrqLkI. I get a little something too.