Google has lost some of your data in the Google Search Analytics report within Google Search Console. Specifically around image search clicks and impressions. The data anomalies page says the issue started on March 24, 2017 and is still ongoing:
Some data for clicks and impressions in image search is being dropped. This is a logging issue, not a traffic issue. We are investigating and hope to have a fix soon.
If you filter your search traffic by "image" results, you will see the "note" show up in the Search Analytics chart:
There is no ETA on when a fix will be released, and I assume the data is not loss forever but I am glad Google documented it.
I don't see many complaints about this within the community, yet.
Forum discussion at Google+.
