An image is nominally a 240 x 240 block. The number 240 was chosen so that the block may be cut into either
a 2 x 2 array of 120 x 120 pieces
a 3 x 3 array of 80 x 80 pieces
a 4 x 4 array of 60 x 60 pieces
a 5 x 5 array of 48 x 48 pieces
a 6 x 6 array of 40 x 40 pieces
(one could in principle do 8 x 8, 10 x 10, etc. but the pieces would be miniscule).
To allow room for the little tabs which link puzzle pieces together, the 240 x 240 block is given a 30 pixel margin on all sides. Thus the final image is 300 x 300. The margin's contents simply repeat part of the central 240 x 240 block. How they repeat it depends on whether the image is for a torus or Klein bottle puzzle. In the case of a Klein bottle, the lines of symmetry will be at x = 30, x = 150, and x = 270.
The puzzle image should be a .gif or .jpeg file. I would be glad to help you convert your image from some other format if necessary.
Please do not e-mail me your puzzle image directly (big files clog up my e-mail queue). Instead please either tell me where I can download your image, or ask me to give you instructions for uploading it to a file server.
Thanks, and happy drawing!