Can I delete photo files from Mac desktop after import to Photo?
OS X (10.10) Yosemite's Photos app stores all photos in its own library.

You can find this library in your «Pictures» folder (/Users/USERNAME/Pictures/Photos Library).

So any image that is imported is stored in that library (unless you have specified otherwise in the Photos app's preferences).



 has made a nice «Get started» page you should take a look at. There it states:


  Photos stores your library in the Pictures folder on your Mac by
  default.




As a conclusion: You can safely delete any image files that has already been imported.



On a side note: You can also view the raw image files inside the Photos Library by right clicking, selecting Show Package Contents and navigating to the Masters folder.