I don't know what "artifacting" means but I've used these same templates to make dozens of calendars for friends and family on my own pc and 2 others. They print out quite clearly for me. The Yahoo site compressed the images to comply with their size restrictions and changing the pixel size to 961x1212 should simply decompress them back to the original template size.
You are quite correct in that there are numerous calendar creating programs available on the market but I simply offered these up as freebies to have some fun with.
BTW, if I might drift off topic a moment, have you ever considered making baseball caps with an Airstream theme to go along with your great tees? I'm thinking something like this would be nice,
although this particular design would probably violate the copyright laws: