The following links deliver a simple application for OSX or Windows that executes the Discrete Cosine Transform on an arbitrary image.
Use File -> Open to open a .jpg, .tiff, or whatever image file you'd like. Hit the "Shrink By 2" button until image is visible in left window. Hit "DCT", wait (the transform is complete when the counter has counted through each row). Note that this may take several minutes. When the compression is complete, the total time in seconds replaces the row count. Hit "Partitions 8" to compress image (256/8 = 32 fold). Hit "Inv DCT" to reconstruct image.