What shortcuts are you referring to?
If you are just trying to get your custom icons in place of the default ones just long press on it and a pop up should come up saying change icon then select custom icon and it will take you to your gallery to pick them.
If you are using a program such as smart shortcuts long press in the space you'd like the shortcut to be select the for ex. smartshortcuts of choice and then follow the same procedure mentioned above to get a custom icon.
If that isn't what you were trying to do a little more detail and I may be able to help you out
I think s/he wants to use an icon pack they downloaded as the icon in the dock. I don't think you can do this. I haven't figured it out yet, anyway