I'm really happy you finally showed up! Hope you had a great holiday! :)
If you remember we already discussed a similar image gallery where the large image appeared in the background. I had no problem centering the background image. But in this new gallery, the large image is a real HTML element as I want to put a caption under each large image and I couldn't do it with a background image.
Your solution has a little problem: the image isn't completely vertically centered. If you pay attention, it's closer to the bottom and further than the top of the container.