There isn't a way to do this, but its not a shortcoming of the app, rather this is the way that Apple determines iOS should work. Icons are fixed in the app distribution from the app store.
What you *can* do is to make a web gallery using Foliobook and then in Safari select the 'share' icon to make a link to the web gallery on the desktop. This will then permit the association of a custom icon based on the image currently viewed in the web gallery.
Couldn't find anything on this but is there a way to make a custom icon for the app?