I think that there is a fine line between photography and illustration. Flipping left to right just about toes the line. Changing the colour of the barn, or inserting farmer Brown on his tractor into the field opposite, or adding flying bids to the sky, would IMO, cross the line and make an illustration of the image. Provided we define the objective and purpose of the manipulation and qualify the result, there is a place for both types of image in art.
Look at the second image in this thread as an example of what I would define as illustration:
Central European Lines