Solved! A right click on missing image symbol in QWebView reviled, that the image had not the address c:/temp/myimage.png as I expected but c:/myimage.png. I've tried to set the baseUrl as c:/temp/. and now everything works!
Looks like QWebView interpreted baseUrl as a html-file, relative to which the images are located. In my point of view it doesn't make sense for QWebView::setHtml
where the source comes from a string, and there is actually no html-file, only base-directory.