Tiling Dogs

You know those people that can draw perfect circles? Well, I just drew a perfect square!! I am trying to work out how to do seamless tiling, which as you can see, I didn't line up properly in a few places. This is a) because my eyesight is failing. b) I need new reading glasses. c) I am working at the wrong scale. d)I don't know what I am doing.
Anyway, I put all the dog faces on one image which was about 1.500 pixels wide and I wanted to select a square from the image so I selected a portion and when I came to resize it to 500 pixels it was spot on...right down to a pixel!!!! I am sure there is something to press that would automatically select a square like pressing the control key or something but I still astonished myself.
Okay, here's a link to a shipbuilder. He probably built the "Mary Rose" and the "Titanic" too.
http://www.livevideo.com/cartoonage.de
If you need a laugh this will do the trick.