Best way round this problem which occurs mostly in IE - and it applies to various other module files as well - is to find in additional_images.php (near line 70) the code similar to :

Code:
if ($num_images) {
  $row = 0;
  $col = 0;
  if ($num_images < IMAGES_AUTO_ADDED || IMAGES_AUTO_ADDED == 0 ) {
    $col_width = floor(100/$num_images);
  } else {
    $col_width = floor(100/IMAGES_AUTO_ADDED);
  }
and replace it with

Code:
if ($num_images) {
  $row = 0;
  $col = 0;
  if ($num_images < IMAGES_AUTO_ADDED || IMAGES_AUTO_ADDED == 0 ) {
    $col_width = floor(100/$num_images)-0.5;
  } else {
    $col_width = floor(100/IMAGES_AUTO_ADDED)-0.5;
  }