Magento: Image not found when I change the values passed into the resize() function.

Question

if I change this:

$this->helper('catalog/image')->init($_product, 'small_image')->resize(80, 80)

to this:

$this->helper('catalog/image')->init($_product, 'small_image')->resize(180, 180)

The image cannot be found, does anyone know why this is?