Asalamualaikum brother my question is : does watcing the nude picture of a gril unintentionally necessitate ghusl if possible show me an evidence if you don't have it's ok

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.

The mere fact of seeing a woman naked does not necessitate Ghusl, let alone just seeing her picture, unless there is ejaculation, as the Prophet said: “Water is from water (i.e. Ghusl is obligatory when there is ejaculation of semen).” [Muslim]

Besides, if looking is without intention, then the person is not sinful if he does not look again and again.

Jareer ibn ‘Abdullah narrated: “I asked the Prophet about a sudden glance at the face of a non-Mahram woman and he ordered me to turn away my sight.” [Muslim]

For more benefit, please refer to Fatwa 360802.

Allah knows best.