Add height of slide show image to javascript


On my website: I have a slideshow.
I noticed that when the page loads, the slideshow is not visible, then everything else loads and the page moves down
to accommodate the height of the slideshow.

I tried adding the height (400px) to the /css/style.css file, with no change.

I was said that I was better off adding the height to the JavaScript.
However, I tried reading the js files with no idea where to add a height.

Thanks for helping.