We can compress png images (especially the once created by using colored scanner) using 'pngcrush', 'optipng' or 'advancecomp' tools.

pngcrush

pngcrush can be used as follows:

mkdir output
pngcrush -d output -brute -l 9 -v file1.png file2.png ... filen.png

optipng

optipng can be used as follows:

mkdir output
cp file1.png file2.png ... filen.png output
cd output
optipng -o7 *

advancecomp can be used in following manner:

advpng -z4 file1.png

advancecomp also seems to give best compression and response time in comparison to others.