That's a pyramid. Advertisers are valued because they give you money, and they will only give you money if they believe lots of Users will see their ads.
Follow the line of thought.
Users will come to your search engine if it's good ... or if it's the only one they've heard of. At this point google would have to become pretty lousy for Users to desert it in droves. You can't make it lousy by locking it out unilaterally. Not unless you get together the top million websites on the planet and get them all to block g### at the same time
:: pause for invigorating chorus of Union Maid or similar anthem ::
so humans are forced to notice that its results stink. Or that its results no longer include anything they've ever heard of.
Not Pygmalion likely. So scratch that idea.
Second line of thought.
Who are the users? What are people looking for when they do an image search?
"I need a good picture of a widget that I can swipe for my presentation/ logo/ own site." (Ages ago I de-indexed some directories because people were only using them for hotlink fodder. People who wanted the page content didn't arrive via image search.) "I forgot what a widget looks like." "I want to look at lots of pictures of widgets." (Follow-up question here: Do you want them all bunched together on a search-results page with no distracting text, or do you want to find a site that includes a number of good widget pictures but also words?) "I want a detailed diagram of a widget with text that explains what the parts do."
What will Users accomplish on your own site that they won't do by looking at your picture on the search engine's site? Is a body count enough, or do they have to spend time and/or money with you?
An overly helpful image search will eliminate most of your warm-bodies class of users. Were they important? (If so, you are probably SOL. De-index your images and move on.) Which others have you lost? Where have they taken their time and money instead?