Many problems of the image management

You set a thumbnail size for featured image but this size is never used in the template :

set_post_thumbnail_size( 600, 220, true );

In the content, you used : invert_standard_img

<?php $thumbnail = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID),'invert_standard_img');?>

Slider :
You do not verify the existence of the slider and thus it creates unnecessarily size

add_image_size( 'homepage-slider',1024,520); //remove if slider is not needed
add_image_size( 'invert_featuredbox_image',250,114); //remove if slider is not needed

And in single page :
You in a var the thumbnail invert_standard_img and use full
Use ‘full size’ in a single page for thumbnail is absurd

<?php
$thumbnail = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID),'invert_standard_img');
?>
<a href="<?php the_permalink(); ?>" class="image">
<?php
if(has_post_thumbnail())
	<?php the_post_thumbnail('full');
?>
</a>