I found that the cause of this problem may lie not so much in a Nextgen gallery stylesheet as in a theme stylesheet (style.css).
In mine I had this entry:
a img, .entry img {
padding: 2px;
border: 1px solid #ccc;
max-width:100%;
}
removing the max-width property solved the issue (though it is still not looking nice because of a messed-up background).
Hope this helps
Paul
