Microwave photons can image a surface by using near-field geometry with spatial resolution close to the nanometer-length scale. We detected electron-spin resonance (ESR) on ruby surfaces by using microwave photons at the S-band frequency (3.73 GHz). The spatial locations of the electron-spin centers were pinpointed with localized incident microwave photons generated by using evanescent microwave microscopy (EMM). We show that the EMM probe is capable of resolving 20,000 spin transitions, compared with the approximately 10(10) minimum detectable spins of the conventional continuous-wave ESR spectrometer. This represents roughly a 6-order-of-magnitude enhancement in sensitivity. Our ultimate goal is to achieve the minimum detectable spin transition of a single electron and nanometer-level spatial resolution by using microfabricated atomic force microscopy-EMM probes.