Hi!
It should be possible to thumbnail a PNG completely from the image stream, using minimal memory (at the expense of speed).
progressive/interlaced images, anybody? still, someone has to contain few lines of pixels.. we've looked at this problem too many times, and still - images have internally to be expanded to full uncompressed bitmap.
Imagemagick is a tool that is a jack of all trades and best solution for none. If the problem is with using Imagemagick to do it, use something else.
use what? current GD maintainer was in our channel few times, telling about future product, that might save us. that future product doesn't seem to have appeared. if anyone wants to revisit GD support for thumbnails, performance et al - please tell your results.
really, this whole discussion is about 'oh, look, these guys have no clue', when we had certain investment of time looking at what can be done.
BR,