That's a great question, Ellen! Honestly I don't have one trick that always works. Usually what I try is either to

1) Type in a very specific description of what I'd like to see on the image (e.g. middle-aged couple sitting on a bench holding hands) and then take it from there, or

2) Describe how people in the image feel (e.g. frustrated freelancer, feeling like a failure)

One thing I noticed is that when you click onto an image on unsplash or pexels is that it's worth then exploring related images underneath. Sometimes I find good cover this way, ones that don't come up in the search results directly.

Hope that helps!