The Best images are the ones you take yourself. Use a digital camera at it's highest resolution and save the Pics at the best quality.
Next would be scanning pics from good quality photos, also at best quality.
Then comes downloads from the net. Try to use pics with the highest pixel counts. They will yield better quality pics.
12" is pretty darn big, even ffor SW plugs. You'll need to set up for legal size paper.
Do it any way you want but fir speed sake, the tutorials method is very fast, and using Devcon will also speed the process as it does cure faster than Etex. Waiting for 2 Etex cures could take days!