a higher pixel camera is always better. its better for editing, printing an resizing. a good size for what you want to do is between 4 an 6 MP. any thing higher would be useless. for what you want to do. its best to re-size a copy of the pic you want to re-size. not the original. that way you can keep the original for editing. and for printing.
here is 2 pictures i took with a hp 3.2MP. an a Nikon 5.1 MP. i resized them to put in here. put you can see the quality difference of the 2