Glad you found it useful!
If you're familiar with screen language (and particular how to use screens), you just need something like:
screen click_to_unlock(): textbutton "Click me to unlock a CG" action Function(bobcgallery_unlockshow, "thecgname")
Which will show a CG named "thecgname" on screen (until clicked to dismiss) and unlock it in the gallery. (Your button will likely actually want to be an imagebutton rather than a textbutton, placed at a specific location. And you'll of course need to show the click_to_unlock screen like any other screen.)
Alternately, if you want to unlock a CG without showing it, just do:
screen click_to_unlock(): textbutton "Click me to unlock a CG" action Function(bobcgallery_unlockonly, "thecgname")