I'm in trouble with finding a way to set DPI value of files within Blender. I have set up an automated workflow to generate image files for printing, but the volume is too high to open and change the DPI value of each image separately. (PNG for example)
May be this addon is the one you need.
Wiki page about this addon.
Note, that this is a feature intentionally left out of Blender, preparing images for print is out of scope for most 3D applications.
Suggest to automate setting the DPI with a command-line tool, eg:
This way you dont have to manually open and resave each time.