I've read that there are no extended licenses, but at one part of this site it says images are usable for book covers. So will my standard license purchase work for a small print run of under 5,000?
Yes you can use them in your book up to a run of 500,000 copies. It is also a good idea to include a photo credit along with any other credits you use in the book. Should you decide to get extended licences though, they are available through Fotolia.com for the same images.
And as a off topic note: As an actor/director/stage manager myself, I'd be interested in your book. Send me details in private message. ;-)