I think the problem is with this line of your CSS:
a{display:block;color: #981793;padding:10px}
The invisible box is the 'a' link element that you have set to display:block which means it stretches to the full width of its parent by default. You need to add a width property to create room for a second one beside it, then float:left on them to bring every second pic up onto the previous line. Just add this after the line I mentioned above: div#content a { width:240px; float: left; }
I had to target the CSS more so that other 'a' elements were not affected, hence the div#content bit.