The computer generation of Scanning Transmission Electron Microscope (STEM) images is one way of approximating controlled experimental conditions. These STEM images are assumed to be composed of signals derived from Poisson distributed variables; three algorithms for Poisson deviate generation are examined. Relationships between the image acquisition parameters and resulting object contrast are given. An application to evaluation of the minimum detectable elemental concentration in Electron Energy Loss Spectrometry (EELS) images is presented. It is found that for typical experimental conditions, the required analysis time for the sample varies approximately as the inverse square of the concentration (when other factors are equal).