There's a couple of persons than could give you better advice but, the first thing is the choice of the pictures you will work with; if they have colors than match, it help, if the artwork of both pictures are comparable it do help as well. If you think than it might help you, there'S a link to 4 of psd files of the fist 4 signatures of my current serie :
http://www.mediafire.com/?z4zuj3hbxdxcma9 .Don't hesitate to ask if you have others question.