HELP! Re: Setting to make a layer appear when user clicks a button inside a lightbox that's linked to another slide

I'm trying to set up a scene where there are two slides.

How I want it to go:

• Slide 1 contains a layer that's hidden when the timeline starts.
• I'm using a slider(bar) for the user to progress through the slide.
• When the user slides to level 1 of the slider, a lightbox appears to play a presentation from slide 2.
• Within slide 2, there's a button which is set to trigger two thing: (1) Trigger slide 1 to show the hidden image. (2) close the lightbox.

 

I'm using variables to change from false to true when the user clicks on a button. 

Every time I try it out on preview, coming out of the lightbox, slide 1 shows the variable changed from false to true, but the image doesn't show.

 

Possible option:

I read somewhere where the designer set the trigger to WHEN SLIDE ENDS, instead of WHEN SLIDE STARTS+conditions. That in fact worked. However, the image doesn't show until the timeline ends. I tried shortening the timeline of the base layer to speed things up but whenever I do so, the image refuses to show.

 

Any ideas? thanks.