ImgOverlay is a simple utility program that allows you to overlay an image on top of your screen for reference purposes. It supports bitmap formats directly supported by Windows including BMP, JPEG, and PNG. Once an image is selected, it can only be moved around or resized if the "Move Image" button is pressed. Otherwise, mouse clicks will go through the image into the window behind it.
This program is very basic and, as far as I can see, complete. Feel free to fork and contribute as you see necessary.