Stack, for those of you who don't ride Western, relates to the length of your jeans. In order to have your jeans fit correctly when you're on your horse, they need to be overlong when you're not on your horse. The act of being in the saddle shortens the length of your pants leg, and without proper "stack", you wind up looking silly as can be once you're a'horse.
Here's what jeans with proper stack length look like when the rider is standing on the ground:
|The right amount of "stack" in your jeans.|
And here's what that (the correct) amount of stack looks like when you're in the saddle:
|Stack gives your pants the right length while in the saddle.|
So while the first photo may look rather strange to the uninformed, it's really quite the right length in order to keep your pants from sliding too high when you're riding. If I had one, I'd post a shot of "what not to look like when you're in the saddle", but don't have such a pic, and am not going to stage one. And for what it's worth, real cowboys don't wear silver buckles they haven't won or earned. That's for folks who are "All hat and no cattle."