The estimation of the low spatial frequency background signals in astronomical digital imagery is a fundamental problem of all the photometric techniques used with such imagery. This problem is particularly acute when numerous foreground sources (stars, galaxies, etc.) are present, because the statistical distribution of background pixel values is skewed. All linear lowpass filters (e.g., the running mean) produce biased estimates in this situation, and the result is biased photometry of the foreground sources, which can produce subtle errors in luminosity functions and other results derived from the photometry (e.g., the apparent number of faint sources may be inversely related to the number of bright sources). These problems can be mostly avoided in many situations by estimating the mode of the skewed distribution rather than the mean. This paper describes a lowpass-filter program which computes an estimate of the mode of the image values in a region around each pixel of an image.