I just tested it now using the latest Custom NPC Mod, for minecraft 1.11, from here and it worked.
You need to go to the gallery, find a skin that you want to use, click to open it, open the share tab and copy the skin url from the skin url field.
remember that you need to change the texture option to url inside more npcs mod.
Below are some screenshots, mines are in portuguese but you do get the idea:
Another option that you have is to download the skins, and put them at a custom folder.
You can add skins to the .minecraft/customnpcs/assets/customnpcs/textures folder or put them in your resource pack in the assets/customnpcs folder
Let me know if you are able to get this working.
By the way, I tested it on a Windows 10 machine, I am not able to test it on a Mac.