Problem cropping an indexed png file

[anthony]

By default IM will not generate an indexed PNG, unless you specifically use a PNG8: format type, and that is equivelent to a GIF 8bit boolean transparency format.

IM can 't always generate the most optimal image file for every image format. Instead you can use other programs like OptiPNG to format the PNG in the most optimal way without data loss, or use PngNQ to quantize the PNG image to a 8bit colored index format (not recommended).

See http://imagemagick.org/Usage/formats/#png_non_im