WordPress comes with the standard theme 2011 – if you are using it and using the great lightbox gallery plugin – images can get hidden under the top banner of your site.
The fix is really easy and just involves editing your style.css file – HOWEVER make a backup and make sure you know what you are doing!
Search for the Header tag and where it says z-index change this to say 99 and hey presto – the lightbox images appear over your header, not under it.
#branding {
border-top: 2px solid #bbb;
padding-bottom: 10px;
position: relative;
z-index: 99;