How to get Product image url from product id or SKU?

(string)Mage::helper('catalog/image')->init($_product, $image)->resize($size)

Example 1:

 (string)Mage::helper('catalog/image')->init($_product, 'back_image')->resize(300) 

Example 2:

(string)Mage::helper('catalog/image')->init($_product, 'image')->resize(250)

Example 3 (origional image without resize):

$_product->getMediaConfig()->getMediaUrl($_product->getData('image'))