Basically, it convines them and starts giving more images with more variations but using the images you chose as a starting point. Just like evolution. The ones you like keep evolving and then you get new ones and then you choose new ones from those new ones and so on and so on. Until you get the images you like and want to use. The user is like mother nature and the images are like species.
Mother nature decides wich species survive and can reproduce and wich ones wont.
The user does the same for the images. Genetic algorithms not only create images, but can also create structures, models, etc. There are other genetic algorithms that don't need input from the user. At least not during the actual run of the algorithm. They work by themselves and the same algorithm decides wich ones survive and wich ones don't.