I realize this is an old thread, but maybe this can help some people. Suprb's suggestion works in concept, but there's no place to add a div around the thumbnails, since those pages are generated dynamically. Instead, try this:
#img-container a {
display:block;
float:left;
width:100px; [this width can be anything you want]
height:100px; [also anything you want]
margin:0 10px 10px 0; [margins for space between the thumbnails]
padding:0;
border:0;
overflow:hidden;
}
This is just telling your links inside the img-container to display block level so you can set their size. Since your thumbnails are links, this style will be applied to them. Then you're telling them to float left so that they stack up inline and look like the normal thickbox thumbnails. There are two potential problems:
1) you have to make sure you set a thumbnail size large enough so that your boxes don't end up with white space inside them. On my site I altered the defaults.php file to include a 300px thumbnail option to cover this
2) this will only show the top left corner of your thumbnails because the overflow is set to hidden, thus hiding the bottom right corner. I tried for a while to center my thumbnails inside these links, but I could never get them exactly right. I tried setting auto margins on my images, but that didn't work. Then I tried setting margins to -50% for the top and left. This moved them close to the center, but they were still off. If anyone has suggestions on this, I'd love to hear it.
Good luck.