Is it possible to add a spinner while loading either the first slide or loading all the slides? Then remove spinner and start the show after the load is complete.
Wondered if the "Before Callback" could be utilized, but not sure how to implement it.
Loading images is tricky at best. You can look into ImagesLoaded, which seems to be the most intelligent and robust approach so far.
I just worked with the before() hook and it seems not to fire "before" the first slide.
Here is an implementation that can get you closer to where you're going on CodePen. Previously, I've loaded the image srcs into data-attributes, and then before each slide, taken that data and put it in the src. There are many ways to work with ResponsiveSlides and achieve whatever you want, but this is a simple elegant project that does one thing really well, and if you need much more - you will find yourself writing a new slider.
Add as background :)