Images are just signals as a function of 2D space. So you can use the 2d FFT: http://fourier.eng.hmc.edu/e101/lectures/Image_Processing/no...

Another idea to grok: a checkerboard pattern image where every other pixel row and column are alternating full black and full white represents the "highest frequency" image possible at that sample rate (pixel density). This is the 2d equivalent of an alternating sinusoid +1,-1,+1,-1, etc