There are many imaging systems. Their performance characterization is important for all applications. Various definitions are introduced for quantification of image resolution, which is the ability of an imaging system to separate two localized signals. In the nonnegative space, we postulated a set of axioms that a good image resolution measure should satisfy, obtained such an image resolution measure, applied our finding in comparing medical CT scanners, and won a 2004 Herbert M. Stauffer Award. We believe that imaging theory can be unified using the axiomatic approach.