[Image-SIG] Image's resolution (dpi)

[Vladimir Ignatov]

Hello!

I need a simple utility that can automate some primitive image manipulations
with PNG files (say, add a 100-pixel colored border around existing PNG
image). I have try to use PIL. It works fine except the one point. Then I
save a modified version of source picture I need to preserve original
image's resolution (dpi). I can't see a way how it can be done for PNG files
using PIL.

Any suggestions are welcomed  (include recommendation for the other
libraries).

    Vladimir Ignatov