Problem: What are the most important words?
Train driving down a train track going under a bridge.
Answer: bridge, track, train

Problem: What are the most important words?
an old wooden bridge extending over a still pond into trees
Answer: bridge, pond, tree

Problem: What are the most important words?
a colour image of a small fishing boat returning to the harbour having just passed under the pedestrian bridge
Answer:
boat, bridge, image