More on the problem: this is theme twenty twelve.
When you open a page with a gallery and click on, say, a thumbnail in the top row (which will be near the bottom of the window), the header is in front of the picture (which is now full size).
If you open a page and scroll down so the first row of thumbnails is at the top of your window, the header doesn’t interfere.
This happened after we installed jquery lightbox for native galleries, but we love the features it offers in addition to the WP gallery.
We’re stumped.
Having the same problem at http://www.1957bhs.com – the only way to get the header out of the way is to have the viewer scroll down so the first line of graphics is at the top of the screen.
If we open the page, and click what we can see (perhaps a line or two of the gallery), the header intrudes.
Using 2012 Theme.
Any help appreciated