Hmm, I haven't see this before. I just tested the gallery in Chrome on both windows and mac and it loaded up correctly for me. I am wondering if its an issue with the image not fully loading and therefor the lightbox thinks the image width is less than it really is?
What if you click on the first image, it loads like your screenshot, then close the lightbox. Click on the first image again, does it work properly at that point?
-justin