[Plugin: Nextgen Gallery] Hidden gallery pic by header with Firefox 38

Hi, i discovered a problem with Firefox 38 and latest NGG Plugin

Please open a picture at this site:

http://www.maler-voelker.de/voelker/referenzen/

Do NOT scroll on the site first,just click immediately on one of the first picture in the first row -> the picture is hidden by header.

In Internet Explorer 11 the problem is not there.

I need a css guru who can fix this ?

Which custom css code do i need to fix the problem ?
I am just a user and no programmer. I see i can add custom css code to my site by Theme Options. I am using latest Avada Theme 3.8.4