High definition photos printing from KDE

[Tulear]

I tried to print (canon i865 with turboprint drivers) digital photos 
(1600*1200 pixels) from KDE applications - mandrake 10.1 - (konqueror, 
digkam, kuishow ...) but result is not so good even choosing the best 
printer definition (4800 dpi) : colors are fine but I can see pixels on 
4"x6" or A4.  
 
It's not a driver problem : printing from Gimp 2.0 (with same driver) give me perfect 
results, as good as from Windows. It looks like KDE applications have reduced the image 
resolution ? Any idea about this curious pb ? 
 
Thanks