Google has recently mentioned that there might be some missing analytics/traffic data in their Search Console. Google reported as caused by a data bug, so there is no need for webmasters to be worried or panic.
According to Google, the impact is on search analytics report commencing March 24th, 2017. There is no actual traffic loss here. Webmasters can identify this anomaly if the filter their data to display only the image search traffic.
At the moment, there is no update yet on when this issue will be closed. The image data anomaly can be further read on Google Note on Search Analytics Report. Click on the image below to visit the Google Note :