On the Mac I use ImageAlpha to reduce the colours to 256 (it has a preview window so you can see if it's OK), then when saving the file I choose the option to s

On the Mac I use ImageAlpha to reduce the colours to 256 (it has a preview window so you can see if it's OK), then when saving the file I choose the option to send it to ImageOptim. On Linux you can get the same by using pngquant, then using trimage. AFAIK there's no GUI frontend for pngquant so I guess how many colours would be ok and check before running trimage. The reason why I use 2 tools is that ImageAlpha/pngquant is lossy and ImageOptim/trimage is lossless.