1. A computer-implemented image processing method, comprising: receiving, using at least one processing circuit, a plurality of image frames of a video, wherein each pixel of each of the plurality of image frames has a blue channel, a green channel, a red channel, and an alpha channel; constructing, using at least one processing circuit, a plurality of statistical models of the plurality of image frames at a plurality of pixel granularity levels, the plurality of statistical models including: at a first pixel granularity level, a spatio-temporal (S-T) histogram for each of the pixels from the plurality of image frames, wherein a first axis of the S-T histogram represents channel value bins, and wherein a second axis of the S-T histogram represents counts of image frames per bin; at a second pixel granularity level higher than the first pixel granularity level, aggregate histograms for the blue, green, and red channels, respectively, based on aggregated pixel values at the second pixel granularity level; constructing, using at least one processing circuit, a plurality of probabilistic models of an input image frame at a plurality of channel granularity levels based on the plurality of statistical models, the plurality of probabilistic models including: at a first channel granularity level, a probability image from each of the S-T histogram and the aggregate histograms, wherein each of the probability images comprises a plurality of pixels each indicating a probability of a corresponding pixel in the input image being a background pixel; at a second channel granularity level higher than the first channel granularity level, compact probability images from the probability images at the first channel granularity level; merging the compact probability images based on a weighted average to form a single probability image; subsampling pixels in the single probability image; determining background pixels, based on a probability threshold value, from the subsampled single probability image; and replacing the determined background pixels with desired pixel values.