[GrannyGrump]

if these illustrations consist of areas of "flat" colors, they can probably be reduced to 8-bit 256-color quite nicely. If gradients, probably not. Or, if absolutely accurate colors are *critical*, you might not want to disturb them. (although colors displayed electronically are seldom going to be very accurate, it is the nature of the beast for different rendering engines to show different results)

You might experiment with converting to jpeg, with a considerable saving in file-size. I did a batch recently. For example --- a smallish illustration --- png 765x1200 px at 1.5 MB--- converted to jpeg with 15% compression (or as some graphics editors dub it, "85% Quality"), resulting in a 280 KB file. It looks pretty darn good, too! (this was an image extracted from a scan of a 1906 vintage book, so it was far from perfect to begin with). I also saved the same image as PNG reduced to 8-bit 256 color, resulting in a 470 KB file --- larger than the jpeg, but still very much smaller than the 24-bit.

But png does give a much cleaner image than jpeg, and doesn't spit stray pixels into adjoining colors. So you juggle your priorities --- file-size vs. quality.

I don't know if the attachments will give a clear view, but it seems to me that under normal conditions, the quality for both is acceptable. Compare to the "Original" (had to crop to meet upload limit), see what you think.