You need to have the picture on the internet so that it has a URL.  Copy and paste this URL in the photo link that appears at the bottom of the dialogue box that appears each time you start a post.

See a few threads below a detailed explanation I have to another provider recently.

Also, read up on the TER Newbie/Self Help Manual which you will find linked on this board for the various rules governing where and when you may post photos as they are considered ads and therefore have to conform to TER's rules.

you can post one ad per week on a regional board.  Some regional boards have theme days such as ta ta Tuesdays, wet Wednesdays, etc. where pics get posted as well.  

Good luck to you.