@wysota:
This doesn't contain anything for a regular JPEG.

@orlando:
You have been misunderstood. The DPI you need is the information from acquiring the image. If you scan a ruler and want it printed out in the same scale, you need to know the DPI of the scanner / camera and the printer.
The information probably is stored in the metadata of your image file (EXIF or similar).
I know of no Qt function to retrieve this information.