I've been a big fan of Pixelmator for a while now. I have it installed on my Mac, iPad and iPhone and it's my preferred image editing app. It's not as feature-rich as Photoshop but it does the job for most of the tasks I need such as removing unwanted blemishes, cleaning up backgrounds and making minor edits. But the new Pixelmator Pro will take advantage of Apple's new Machine Learning capabilities.
Pixelmator Pro will clean up some of Pixelmator's more annoying features. For example, every time I use a different tool on my Mac, I end up with another floating palette on my screen. Now, the tools and their various customisations will appear as part of the main window.
RAW editing will also be added - not having it in Pixelmator is a significant omission in my view - and there will be a bunch of new visual affects as well.
New tools, that take advantage of macOS High Sierra's new Core ML framework, make it a matter of swiping across the display to remove artefacts in your images. There's a neat animation on Pixelmator Pro's Machine Learning page that shows this in action.
There's no word on pricing for Pixelmator Pro. Pixelmator for Mac costs $46.99 in the App Store. In comparison, the cheapest way to get Photoshop is to pay $14.29 per month for the Photography Creative Cloud package. I'm guessing it will be around the $65 mark - making it an inexpensive initial purchase or update while still undercutting Photoshop by a good margin.
If you don't want to spend anything, there's always the open source application GIMP. It will do the job although I've never liked the user interface.