First you need the current sources from SVN repository.
Then you can use this code:

var
  XRes, YRes: Single;
begin
  ... load image
  if GlobalMetadata.GetPhysicalPixelSize(ruDpi, XRes, YRes) then
      ... do something with DPI info
 

If GetPhysicalPixelSize returns False that means no DPI info was present in the last loaded image. Also note that currently DPI info is read only from JPEG, PNG, and TIFF images.

how i can get the dpi of a image?