What do you call the artwork you first see upon entering a place

Solution 1:

According to my wife, who is trained in Art History, this is a marquee piece from the word marquee meaning a sign placed at the entrance of a building.

Solution 2:

Consider the term "centerpiece"

(n.) an item or issue intended to be a focus of attention: the tower is the centerpiece of the park.

Definition from New Oxford American Dictionary

The Fénykövi Elephant is the centerpiece of the Museum of Natural History.

Solution 3:

If this piece of art puts you in the right mood for the rest of the exhibits, if it has the same functions as a starter course in a meal you could say:

The Fénykövi Elephant is the tone setter art piece of the Museum of Natural History.

Tone setter a person or thing that establishes the quality or character that is to be followed subsequently

or, more figuratively, borrowing from music and drama:

The Fénykövi Elephant is the opening act of the Museum of Natural History.