I'm not sure if it's bug, but it seems to me. I work on site that does not have IH installed but still uses large images. Unfortunately in cases where there is more then one image thumbnail is displayed for additional images.
As far as I can tell $flag_has_large isn't set in 'additional_images.php' on line 91. Quick fix is to replace:
PHP Code:
if(defined(IH_RESIZE))
        
$flag_has_large true;//file_exists($products_image_large);
    
else[...] 
with
PHP Code:
$flag_has_large file_exists($products_image_large);
if(
defined(IH_RESIZE))
        
$flag_has_large true;//file_exists($products_image_large);
    
else[...] 
Of course, I can be wrong ;)