Though this postcard is British, I believe the Dakota on the front is a reference to the subject, the Black Hills of South Dakota. And it appears that the landscape is painted in oils rather than watercolor, which is more typically the case. Quite a nice little painting I think, c. 1910, and a perfect English-American hybrid!
3 1/2" x 5 1/2" and in very good condition, with a little paint staining (not foxing) on the unused back.