adding a catagory to pics

Hi, i'm new here so maybe its obvious, but i cant figure out how to add a catagory to my pics. the first one i uploaded i forgot to click on so there isnt any at all and i cant figure how to go back to add one. and is there a way to add more than one catagory to a pic? after clicking on one, it wouldnt let me choose more even if i held down the control key while clicking, and once i chose one and hit save or upload, that was it and i couldnt go back to add any more. thanks!!