Quite honestly, Costco's machine does a remarkable job of blowing up a shot. I printed a 6mpix pic off my D70, edited in Picasa no less, and the 12x18 was razor sharp. I didn't even bother resizing it to 12x18 300dpi or whatever. I just uploaded it as a 2.5MB file.
Needless to say, I am very impressed with the quality and the price. Of course, pros can be more picky, but they more than satisfy my needs. Turnaround time is very fast too. I uploaded shots at midnight and get emails by 10am the next morning saying they were done. I mean its so cheap that you can just trash the pic if you don't like it.
You have a couple of photos that are great... you must be very good at photoshop!