I had a fun job recently to paint a landscape for a book cover. I won't reveal the title until the book is published. The assignment was to paint a specific place but there was not a good source image to work from. I had to assemble this scene with room on top and bottom for type. Being able to paint a landscape so it looks like it's an old painting is harder than just painting those colors fresh. Painting a blue sky in a faded and yellowed key is a green. You have to know that in the painting that color is blue. That takes some trust and restraint.
Finally, to complete the vintage look I applied cracks on the painting. This is always a crap shoot for me. I marvel how Marc Burckhardt http://www.marcburckhardt.com/ can get such a good reliable crack pattern when he does it. Mine is always a mess of a pattern but it works.
My hint for doing these kinds of scenes is the use of golf courses for reference. The neatly trimmed grass often meets a wonderful bank of trees that are easy to see and provided they are the right kind of trees.
I hear this book is one of a series so I look forward to going back in time again soon.