Here's a quick sketch example. Really it's mostly that the poses feel too stiff. When a character interacts with a static object it causes a major shift in weight, so really try to show that with the ghost girl leaning on the bench or the boy musician leaning his arm around the banjo. Rarely are there any horizontal straight lines in human poses, so watch what your shoulders and hips are doing at all times! : >
Also to create a little more movement in your shot, try to pump up those angles of perspective on the bench. Straight horizontal lines in the middle of the frame doesn't create as much interest as a diagonal line would. It's a strange quirk of the human eye artists like to take advantage of, so push the perspective some!
So, I would totally get some gesture drawing practice with poses too just so you get a bit more familiar with relaxed poses. Live/photo reference is your friend. : >
(Oh, and I just drew ghost girl with swirl legs since I wasn't sure if she had any. XD And I realized too late banjo boy had short sleeves. Oh well.)