While they aren't as crazy powerful as Photoshop for complete image editing, they do offer most of the basic features we use photoshop for in the first place. Image retouching, levels, background separation, etc.
Affinity Designer is a pretty solid, lightweight program and is only $39.99.
Pixelmator seems to be really great at all of the photo manipulation stuff as well and is only $29.99.
Combine using these with Sketch for UI design, and you may have what you're looking for.