Although it is okay to manipulate images to learn Photoshop, that
should not be considered original art. There are actual programs
designed to scan remixed images to find matching sources, it is
copyright infringement. That being said, once the ball is rolling,
there are many ways to source original images (cell phones, digital
cameras, scanning objects, etc.) or use copyright free images for
practice from places like Creative Commons. It depends on where you
are in your program & how the art will be presented (as original art,
portfolios, exercises, practice).
Politics--I read the WSJ article & did not feel upset by it, rather,
I appreciated that perspective being brought to my attention. Artists
are generally very open-minded and accepting, and I was reminded that
I need to be aware of other perspectives, too. Let's just teach great
things in great ways, and let our legacy speak for itself--the journey
is the point & we are all on it together.