How to make custom callback in keras to generate sample image in VAE training?
I think the two following links could help you 1, 2.  

The first one is a tutorial, which introduces you how to display images in TensorBoard.
If you look at the part on the confusion matrix, you should find a way to make your desired callback, i.e. a callback which involves displaying an image.  

The second link is also a tutorial that shows you how to write your very own custom callback in TensorFlow with Keras.  

I think by combining both tutorial you should have a pretty good idea how to do what you want.