Cleaning my photo library, but is it really on MAc OS?

Hello:
My Mac OS drive is out of space, and i need to remove the unneeded things, it told me that photos take 20Gb, but there is no way to delete them,
first of all, i do not think that i stored any photos on my Mac Os drive, so, how is that? and, how can i clean it? all my photos are outside of my Mac OS at all,
any help will be highly appreciated