(This goes for all mods, not just on Macs, Windows as well)

Before installing the mod, create a new folder within the images folder - call it 'Backup' or similar. Select all the files in the images folder, then choose 'copy' then paste them into the new Backup folder.

Then install the mod to the images folder. You will be asked if you want to replace existing files; say yes to all. Open the game and your mod should be working.

If/when you want to remove the mod go to the Backup folder, select all, copy, then paste the files into the images folder; again, say yes when asked if you want to overwrite the files. Your game will be back to normal when you open it again.