Hi STefanie.
Its a very good idea, and one we have looked at before. In the old version it was not possible, because those images were shown in whats called a ‘random image slideshow’ 🙂 .. which didnt allow for writing undertext.
It may be in this new updated version that its possible, I still dont know, – am still getting to know it 😉 …. but it will be one of the things I will put on my list of ‘to do’es’ …