I guess I must be a particularly expert googler, because I found pictures of twilight skies by using...<twilight sky> as a search term.

When you have terms that have been used in popular contexts that skew your search, simply add an adjective or noun that would narrow it down.

Easy peasy.

BTW, after you have a page of suitable images, you can also narrow it down by selecting one of the image options at the top of the page. E.g., "high resolution," "purple," "sunset," "stars" [etc.]

And then there's Advanced Search, which allows you to include any number of qualifiers for the particular kind of image or image file you are looking for.

So you see, it's not Google that has the problem. The digitized brain works just fine, and has taken all that you mention into account.

If it's on the web, I can find it 99.9% of the time.

All you need to do is stretch your brain to accommodate google's genius. Because it really IS genius!