I like your button selection, too many people choose 'shift' as a primary key, which in windows turns on sticky keys.
I also like that your sprites have a fair amount of animation, especially for a first test game. The cow carcases as save points is also a nice touch.
If the object is to collect sheriff badges, perhaps have some way of keeping track of how many you've picked up/how many are left to collect in each level.
Perhaps some basic sound effects and a background tune...although sound doesn't always work the best in HTML5 games quite yet :)
Overall though, a solid first attempt.