Maybe the following problem is helpful to avoid conflicts between other contao extensions:

I install "contao-lazy-images" and "contao-bootstrap". I enter "img-fluid" into the backends expert>class>field of an ce_image element. The extension "contao-bootstrap" usually takes the class "img-fluid" from the ce_image deeper into the DOM and moves it directly to the img class. But the "lazyloaded" class seems to be stronge ;-). The "img-fluid" doesn't occure, neither on the ce_image element nor on the img element.

I deinstall "contao-lazy-images" and the "img-fluid" is added as expected to the img class.