Training AI to do repetive retouching job
This is a very (very) broad question.

What you are trying to do is utilize some neural model to perform image processing. So your output needs to be the same size as your input, meaning that you need to use a fully-convolutional model for this.

I suggest you read the following article:

Fast Image Processing with Fully-Convolutional Networks

They are doing exactly what you need, only for different image-processing methods (and more than one).

However! I think that before going on to implementing a neural model, you should read a bit about the general subject of machine learning and deep learning. How Do I Learn Neural Networks?