Hi,
it only shows a black square
That can happen when the hotspot image is larger than the maximum texture size that a device support.
On older or lower-end mobiles this can be 2048x2048px.
So if you keep the hotspot images below that size you're safe.
But it's already on my list to check automatically for that size limit and automatically downscale the hotspot image at runtime to avoid that problem. Then the hotspot loading will be slightly longer and the hotspot maybe slightly blurred on such devices, but it will work.
Best regards,
Klaus