I used Photoshop 7 with a Wacom Intuos 3 Graphics Tablet. The original image was done at 21,600 x 14,400 pixels, allowing for more detail. Sadly, working with an image that size is hard for my computer to handle, and I couldn't get it to convert to .png to be uploaded to my Zazzle.com account until I reduced the size to 10,800 x 7,200 pixels. The good news is, that's still 300ppi on a 36" x 24" poster/print.
Thanks for the comments and questions.