Why can I not use xdg-open and gnome-open in Mac?
If you're trying to open an image file like a .png, .jpg, etc. from Terminal in OS X, you can use open filename.png where filename.png is the actual pathname of the target image file. By default, it will open in Preview.

In Terminal, type man open and press enter to read the BSD General Commands Manual for the open command.

OS X is not Linux and Linux specific commands like xdg-open and gnome-open do not exist in OS X unless they've been ported by managers like Homebrew and you have installed using one of the managers like Homebrew or installed from source code by yourself, that is if you can satisfy the dependancies involved too (which is what Homebrew does for the command it ports).