Put the lightbox thumbnail and the gif thumbnail preview in a DIV
Set the gif to none (hide it)
Apply a mouse hover animation to that DIV so that when you hover over it, it shows the gif thumbnail
I’m not sure it would work without the lightbox, but if you don’t want them connected, you can make them all separate (unlinked) lightboxes. Here is the project I was messing around with this idea on - feel free to check it out as I’m not an expert and very well might have missed a step
just a little heads up - you need to toggle the preview off then on again for this to work, not sure why that is happening