Thoma's English Wikipedia entry is here, but for more information I suggest you click on the left-hand panel, select Deutsch, and have it translated if you don't know German. Otherwise, a bit more biographical information in English is here.
He was born in a small town in the Black Forest, far from from Munich and other art centers, and more than 30 years before Bismarck created a unified Germany. Yet he was able to work his way up from decorating cuckoo clocks (an important Black Forest product then and now) to eventually becoming a professor at the Grand Ducal Art School in Karlsruhe and director of the Staatliche Kunsthalle Karlsruhe, the state art museum there.
I suppose he had a reason for showing red paint on his brush, because almost no red can be found otherwise.
Painted when he was 60.
The man and his dog in the foreground are overwhelmed by the rest of the painting, including much sky and its clouds.
Here he was capturing the darkening leaves.
There are sheep with their owner's mark painted on them, a woman in the distance apparently doing some work, and some young people in the foreground. These latter are jammed at the bottom of the canvas, the girl's dress and feet being clipped off, as is the left foot of the boy playing a fife. Those figures and their activities (such as they are -- the girl seems bored) are somewhat hard to explain other than they might have been dragged there by the distant women and ordered to entertain themselves until she was done with whatever she was doing.
Diana was the goddess of animals as well as of hunting.
Interesting contrast between the craggy face and the smooth background.
Thoma painted a number of religious-themed works.