Problem: What are the most important words?
travelers and several taxis waiting in front of the railway station
Answer: taxi, traveler, wait

Problem: What are the most important words?
silhouette of profession fighting a raging fire
Answer: fight, fire, profession, rage, silhouette

Problem: What are the most important words?
Two French school girls position a slide under a microscope.
Answer:
microscope, position, slide