They extract concepts from their training data and can combine concepts to produce output that isn't part of their training set, but they do require those concepts to be in their training data. So you can ask them to make a picture of your favorite character fighting mecha on an alien planet and it will produce a new image, as long as your favorite character is in their training set. But the extent it imagines an alien planet or what counts as mecha is limited by the input it is trained on, which is where a human artist can provide much more creativity.
You can also expand it by adding in more concepts to better specify things. For example you can specify the mecha look like alphabet characters while the alien planet expresses the randomness of prime numbers and that might influence the AI to produce a more unique image as you are now getting into really weird combinations of concepts (and combinations that might actually make no sense if you think too much about them), but you also greatly increase the chance of getting trash output as the AI can no longer map the feature space back to an image that mirrors anything like what a human would interpret as having a similar feature space.
You can also expand it by adding in more concepts to better specify things. For example you can specify the mecha look like alphabet characters while the alien planet expresses the randomness of prime numbers and that might influence the AI to produce a more unique image as you are now getting into really weird combinations of concepts (and combinations that might actually make no sense if you think too much about them), but you also greatly increase the chance of getting trash output as the AI can no longer map the feature space back to an image that mirrors anything like what a human would interpret as having a similar feature space.