Preview: How get rid of white space above/below screenshots?

My objective is to be able to make a composite screenshot by opening several and arranging their images on top of each other without unnecessary white space at the bottom and top of each of the pictures.

I did not have such top and bottom white space when I used Preview with Leopard 10.5.8 on an old G5, only now with Lion on a mid-2011 iMac.

Your counsel, please?

Respectfully, Norm