My question is this; how do I price my images for sale?

Is there a rule of thumb that I should be following?

for example, a full color poster having a size of 18 x 26 inches. printed on photo stock type paper, should sale for how much?

Here's just one of the images that I'll be selling: "Ms. Maddy Hatter"

http://excalibur.renderosity.com/mod/gallery/index.php?image_id=1239279