Hi there,

Is this in one particular widget or all of them. People have reported this issue happening randomly since the last WP update, the dev team are looking into it. Make sure you’re on the latest release, WP 4.5.3, this latest release seems to have resolved that issue.

If not, go into the media library, copy and paste the media file url into the image field in the widget and then save to see if this resolves.