In order to unlock and view all the endings, you must do the following.

Samus at a bar in a red top and shorts:
Beat the game under 2 hours and 100% in Normal.

Samus in a red top and shorts, walking around a city:
Beat the game over 2 hours and 100% in Normal or Hard.

Samus in full armor sitting on a rock:
Beat the game in Easy or in over 4 hours and above 15% in Normal or Hard.

Samus in just her blue jumpsuit:
Beat the game under 2 hours and above 15% in Normal and Hard.

Samus over looking a city in a red top and shorts:
Beat the game under 2 hours and 100% in Hard.

Samus sitting near a window in a red top and shorts:
Beat the game below 15% in Normal.
Samus standing in full armor, with a close of her face without it in the background Beat the game below 15% in Hard.

Samus without her helmet looking at the sunset: Beat the game around 3 hours and above 15% in Normal or Hard.