In particular, notice that I suggested the extension .tmp (or the double extension .png.tmp). This way, if the computer crashes after pngcrush created this file but not yet cleaned it up, so this file will be left over, the end user or various cleanup programs would know that this file is temporary and can be manually or semi-automatically deleted (in the vein of `find ~ -iname \*.tmp`, followed by `find ~ -iname \*.tmp -delete`).
