Any reasonable img lazyloading implementation should produce plain img tags once loaded. Not sure why you would end up with background-image’d block elements.
If it failed to load, it would leave behind an img tag that failed to load, like any non-lazy-loaded image. Unless you’re talking about JavaScript code failing to generate an img tag (e.g. from a data-src attribute), which would be bizarre.
Edit: by “once loaded” I meant once loading is triggered, if I wasn’t clear enough.