Given a GIF, JPEG or PNG image, this will shrink and dither it down to something resembling what it would look like displayed on the screen of a Commodore 64.
This is pretty close to what could be done on the C=64 using the FLI vertical-sync trick; however, this is a bit of a rough hack. ConGo provides a much more realistic rendition, if you want verisimilitude. The palette of colours available was taken from this page.
Source: c64ize.php, background.gif, c64_palette.gif, GPL, requires ImageMagick, is a hack.