I have a heightmap file in .png format. Is there anyway to use this image?
Only if you convert to a .raw image for can be used by Unity.
With IrfanView you can save as a .raw image but first install IrfanView(http://www.irfanview.com) then install plugins(http://www.fosshub.com/IrfanView.html/irfanview_plugins_441_setup.exe)