Metasynth has always been something you've had to purchase and it was exclusively a Mac app. (my favorite audio program btw)
i can't recall the name of the Windows program you're referring to..
i remember coming across it (or something like it) when reading about Aphex Twin, although i know he used Metasynth, but the person that uncovered it had a free Windows app...
i did a quick Google and found this link~
and here's the pic
although this isn't much help since this is only a spectrogram... kind of the opposite of what you're going for. but another Google search turned up this~
it says its a "FREE Windows program for transforming any picture into a sound"