Custom Field For Tags?

Here’s a concept: I would like to have an image for some of my tag pages. Let’s call it “tagname.jpg”. It will sit in a special place on my tag pages.

Problem is 95% of my tag pages will not have the special image so I was thinking the best way to handle this would be to make a custom field called “tag image” and add an image to it for cetain tag page.

Is this possible? Anyone know how?