You can put the link to the picture in the ad, after uploading the pic to a hosting site such as Tinypic, or Imgur, etc.
Or you can tap the insert image icon in the tool bar at the top of the post, then drag the image from its location on your phone into the box that opens in the post when you tap that insert image icon.
That's two ways, you can read about others in the trading post section in the sticky thread at the top that explains the trading post rules, Titannv put picture directions in there recently.