Photoshop would work well to crop the image. As would Google's Picasa, if you are looking for something free.
You want to start with the original image, and then "crop" the image so that there is some space above his head...like the sky. Also, notice that the avatar is a square here at Trulia, so when you crop the photo, crop it in a square aspect, rather than the normal photo aspect of 16:9. If the original image doesn't have much sky above his head, consider a different image.