Oh Free Software, how brilliant are you? I was looking for an online PNG (oronounced “ping” graphics format optimizer, to squash down my delightful but chunky graphics. I know from somewhere that you can reduce the number of colours in an image and keep the alpha channel transparency. For a start, Fireworks give you this functionality out of the box.
On my search, I came across OptiPNG, a command line utility for doing just that. A skip and a jump over to Synaptic to install it, and then I find Trimage lossless PNG & JPG compressor, pngnq PNG quantizer and pngquant PNG quantizer & ditherer.
I installed them all. Just like that.
And then I tried them all out, and on this occasion, the low-level grey image above, with an alpha channel, compressed by about 40% using OptiPNG.
Postscript: Many of these utilities are available for McWindows too. Give them a go, if image compression floats your boat.