You can reselect a picture once you've placed it by pressing and holding down the picture. After a moment, the blue border and resize controls will appear, and you can manipulate it as you'd like to. To delete the photo, hold it down for a few seconds until the Delete option appears.

(We know that it's easy to accidentally tap the photo thinking you'll select it and leave a spot of ink there. Sorry. :( We want users to be able to fully write on photos, though, which includes letting you dot your "i"s :) )