That's a nice dramatic cover on this issue of COWBOY STORIES. I don't know the artist, but he did a good job. Other than James P. Olsen and S. Omar Barker, this issue doesn't have many well-known authors on hand. The other stories are by Jay Kalez, William E. Brandon, Eben Garrett, Victor Kaufman, Paul R. Morrison, Butler Van Steenbergh, Carl F. Happel, and house-name Ken Martin. It's understandable if you're saying "Who?" in response to most of those. Doesn't mean the stories aren't enjoyable, though.
UPDATE: This cover is by Tom Lovell. Thanks to those who identified it as his work.