I’ve mentioned PNGGauntlet many times on this blog – it’s a great tool for squeezing the best compression ratios out of the PNG format. It combines the tools PNGOUT, OptiPNG and DeflOpt in a nice WinForms GUI.
As part of the compression process, most chunks are removed by default. This includes the pHYs chunk, which defines the DPI/PPI of the image – an important property when dealing with WPF. To make sure this chunk isn’t deleted, add ‘pHYs’ under ‘Chunks to keep’ (PNGOUT) and check the ‘Preserve PNG metadata’ box for DeflOpt.