Although I see the problem in your gallery in Chrome 25, I have been unable to replicate the problem in a test gallery of my own (using the same configuration options that your gallery does).
Unfortunately, it might take some trial and error to try to track down the root of the problem.
Try using smaller images (filesize) in your gallery. I notice that several of your main images are over 1MB in size. When creating a gallery with JuiceboxBuilder-Pro (using the default settings), images are usually around 120KB each. You could also try changing the imagePreloading configuration option. (See the Main Image section of the Config Options page for details.)
Another thing to try would be to change the values of imageTransitionTime and displayTime to see if this makes a difference.

Hopefully, one of the above suggestions will help. However, if the problem continues, then try reducing the configuration options in your gallery's XML file, one by one, until the problem disappears and you find a configuration option which is contributing to the problem.

Problem solved!

It turned out I had forgotten to upload the "twelve" theme, so it was using the "classic" one.

Thanks a million!

Marc