Well one way to do it is to use QWebEnginePage::runJavaScript but be careful it executes asynchronously!
http://doc.qt.io/qt-5/qtwebenginewidgets-qtwebkitportingguide.html

then in your javascript - find IMG element by id/class/whatever and set its contents like an embedded image e.g. change src to

src='data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg=='

this is example 1x1 pixel black dot image you'll have to substitute with your own image data base64 encoded and of course change image format if you're not using png to data:image/jpg(whatever)

The workaround that appends a suffix to the requested image does the trick:

image.source = "image://images/myImage?id=" + Math.random()

but it's quite dirty. I wonder if there is a more elegant way (i.e. using signals) to keep the image source updated.

@SGaist yeah, thanks I tried it and it works now.