Click on the small blue picture icon above the text box, locate the picture on your computer you want to post, then click choose! That's it. PLEASE keep your pictures to no more than 1000 pixels wide with a PPI between 72 and 100. Large pictures slow the forum down and if they are too large the forum will not upload them. Only pictures with a .jpg file extension can be uploaded. The forum dislikes some mobile devices so it is best to upload from a computer.
If you are unsure about adjusting picture sizes Bill has provided a great tutorial here:http://www.craftsmankituniversity.com/vanforum/index.php?p=/discussion/174/picture-resizing-tutorial#latest