YIPPEEEE!

I think I solved my problem!

(Did I just say "Yippeeee"? I haven't said that in years.)

What I did was change main_product_image.php.
There's a line which reads:
$products_image_large = DIR_WS_IMAGES . 'large/' . $products_image_base . IMAGE_SUFFIX_LARGE . $products_image_extension;
Which I changed to:
$products_image_large = DIR_WS_IMAGES . '' . $products_image_base . $products_image_extension;

Bad fix, I know, but it's on the way to correcting the problem. :)