Very interesting! Putting the stone images outside the program enables experiments like that. Here is the comparison:

Not sure for the moment if I should add the adjusted stones to the distribution or link them in the resources page (or replace the current ones). Here is the xnview script:

#start of xnview script (adjust.xbs for instance)
gamma( 1.40 )
contrast( 35 )
settings( 0 0 0 0 0 0 0 )
output( png 0 6 )
#end of script

Thanks for sharing.