According to Wikipedia, “An oil painting is painterly when there are visible brushstrokes, the result of applying paint in a less than completely controlled manner, generally without closely following carefully drawn lines.” It also describes the effects created by a new batch of apps that can take a photograph and make it look like an oil, a watercolor, or an acrylic painting.
Another option is using Photoshop to create a painterly effect. Here are 10 tutorials that demonstrate how.
And last, but not least, some cameras have built-in painterly filters. I created these three images (cat, hibiscus, rose) using my Samsung NX300.