Google said on April 6, 2021 it made a change to how it reports within Google Search Console’s performance report for image search impressions. This change is a reporting only change and there was no impact to your actual rankings in Google Search.
What changed. Google said it “improved the algorithm for counting image impressions in Search results.” That basically means the math it uses to display impression metrics in the performance report was changed to be “more accurate,” the company said.
What’s the result. Google said this may lead for you to see a “small drop” in your image search impression data within the performance reports. This is specific to how Google reports on the image search impressions.
What Google said. Google wrote “Search Console improved the algorithm for counting image impressions in Search results. As a result, you may see a small drop in your image impressions in Search. This is not a drop in actual impressions, just a more accurate calculation of image impressions when type=image is specified in the Search Performance report.”
No ranking change. This does not mean your rankings changed, this is just a reporting change.
Why we care. If you see a drop in your image search impressions starting April 6th, you can likely attribute that drop to a reporting change and not any ranking change. Of course, you will want to track those impressions going forward to ensure your actual traffic from Google has not changed.