This reproduces for data URL images as well as normal images. This may have regressed with recent changes to adopt WKWebInspectorView on Mac port.
<rdar://problem/35176608>
Created attachment 327769 [details] [PATCH] Proposed Fix
Comment on attachment 327769 [details] [PATCH] Proposed Fix r=me
Comment on attachment 327769 [details] [PATCH] Proposed Fix Clearing flags on attachment: 327769 Committed r225234: <https://trac.webkit.org/changeset/225234>
All reviewed patches have been landed. Closing bug.