Is using Views overkill to display a single image above the footer of all pages on my site?
If you need a static image, just use a block.  If you want a slideshow or something, Views is better.

Unless your site is going to get a huge amount of traffic, I wouldn't worry about the performance differences between these two options.  The main concern is how much harder is it going to be for you to set up a view versus setting up a block.

I don't know why you would created a content type and field specific for the image.  If you just want to stick a piece of content in the same place on every page, that is one of the things that the blocks system was designed to do.