My movie files contain title images but they dont show in stacks, but they do in folders. How do I get them to show?

Drag the folder containing your movie files to the Dock to the right-side of the Dock divider. This creates an alias back to the original folder. Right click on that Dock icon and you will see the option to enable the Display as a Stack and the ability to set the view style that you prefer.

You could have found the answer yourself by typing stacks into the Finder's Help search facility.

The following is the setting for the Downloads stack next to the Trash in the Dock.