In the Name of Allah, the All-Merciful, the Mercy-giving

All praise is due to Allah. Peace and blessings be upon His Prophet Muhammad.

First, congratulations, brother, for Allah’s grace upon you by guiding your heart to His true path.

Second, intentional imagination of nude persons is a sin that one needs to give up and repent sincerely to Allah from its commission. It is basically a minor sin, but insisting on the commission of a minor sin without repentance renders it a major one.

However, to imagine a person nude, one needs to have a mental image of this person. Since the questioner has ever seen the Prophet’s Companions, let alone the Prophet (peace and blessings be upon him) himself, how can he imagine them nude?! Most likely, these are mere whisperings from the Satan.

So, the questioner is advised to seek refuge with Allah from such whisperings. He is also advised to implore earnestly to Allah to help him get rid of such imaginations and grant him sincere repentance.

Almighty Allah knows best.