Highlights and shadows are how we see shape, volume, and texture.
Compare the two Picasso paintings below.
One has very little volume, the other, much more.
If your photograph needs more shape, volume, or texture, add some chiaroscuro.
Add highlights and shadows.
You have to study the light source in order to add highlights and shadows.
Highlights are "attracted" to the light source.
Shadows "hide" from it.
A small light source produces a small highlight.
The shadow is dark with sharp edges.
A large light source produces a large highlight.
The shadow is bright, with fuzzy edges.
If the light source is high, the highlight is high.
The shadow is shorter and fatter.
When the light source is low, the highlight is low.
The shadow is longer and narrower.
There are three types of shadows.
The side of your subject opposite the light source is in shadow.
The subject casts a shadow away from itself.
A drop shadow appears behind and below text, banners, and the like.
We won't use drop shadows here.
Airbrushing may be useful if you have a pressure-sensitive tablet.
If so, select Airbrushing in options bar/Tool Options of the Brush tool.