Ok, I understand.
In fact, It is not a bug: each time a slide is displayed, the background is restarted, so yes if you use a cloudy background without any transitions, it won’t be seamless.
Why do we do this way?
Because if you changed something on the background settings (let’s say you changed the cloud density) between slides, we must reload the settings you have chosen and re-initialize the background with the new settings.
So, you are right, actually the best way to display your pictures is to put all your pictures in the same slide and use the delay and duration setting.
I will take a look to this problem and check if there is a way to have a seamless transition in the case where you have exactly the same background with the exact same settings, but I cannot promise I find a quick fix for this issue because it is not the way the internal system is working right now.