Switch Between Photos with Dialog

Yes, the "flash" is because the photos in the custom states have Fade entrance and exit animations. 

• Those animations can be edited via going into Edit States mode, and selecting the pic in the custom state. 
• However, those animations provide the smooth transition between the custom states. So if you delete them, the pic will cut between states (instead of fading between states). 

On the "Start" slide, you could use clean copies of the pics with just a Normal state. And you could start with a Normal (and unanimated) state on the conversation slide. However, starting with Normal will mean there's a flash when the pic first changes to a custom state, because the Normal state will exit without an animation, but the custom state will appear with an animation.  

So, if you're going to use two slides, you'll have to have a flash either when the slides change or when the pics change states for the first time. 

I suggest you use only 1 slide, like you did in the original version. But put the Start button on the base. And add a trigger to it that changes its state to Hidden when it is clicked. 

BTW, in the custom states, the original pics have been deleted, and copies have been pasted in. Unfortunately, that makes it look like you haven't selected the object, even when you have. You can verify that it's selected, because it will be highlighted in the Timeline. And its states will show if you're looking at the States panel.