Allah has made lowering one’s gaze and guarding one’s modesty the strongest means of purifying one’s soul. He explained that refraining from shameful deeds purifies one’s soul.

“Say to the  believing men that they should lower their gaze and guard their modesty, that will make for greater purity for them, Allah is well aware of what you do.” {Nur : 30}

The person whose heart remains attached and enslaved with the image of a woman he saw, he will combine all kinds of evil and corruption, to an extent known only to Allah. Even if he does not commit the major sin of illicit sexual contact, his heart will be attached to her, and this is worst than committing a sin and then repenting from it, whereupon all trace of it is lifted from his heart. These lovers of images are those who will be most severely punished and least rewarded.

(extracts of U’budyyah by Ibn Taimiyyah)