Comment by GrayShade

Seems like there's a race condition between the images loading and the script setting up the sliders; if the script runs before the 'before' image for a given slider has finished loading, that 'before' image won't be visible at all. Happens under Chromium-based browsers too.

Thanks, I was wondering if this was some rich high-quality screen thing I was too poor to understand ;)

Worked for me in Firefox Android (the race condition theory seems likely).

Zooming in and out worked on my iOS device to trigger it to load properly.