Once you have the photo uploaded either in your steam gallery or on a website like imgur, just copy the link to the said photo and paste it in the message between img tags. Like [i m g] link_to_photo [/i m g] without spaces in the img of course (imgur gives you the option of copying a link already framed by the right tags, so you just need to paste that). Alternatively you can click on the button with the picture of the tree just above the space where you write and just paste the link there. It will insert it automatically.