you can use ORDER BY RAND() in your statement but since the WHERE clause is worked first I guess you are right. And ORDER BY RAND() seems not to work in all MySQL/PHP version combinations.
Another thing: I just try to implement your Addon and found massive problems.
First thing: you have one slash too much in your thumbnail path: not /thumbnails but only thumbnails.
Second thing: with display:block the mosaic is shown, but it runs out of the <div> where it is sitting within. Without the display:block the width and height of the <a href is not considered.
Third thing: you use float:left. You have to use a <span> with style="clear:left" after the mosaic block, otherwise the following elements will be mixed up