Will he who looks at naked pictures of women and keeps them marry the ḥūr al-ʿīn if he does not repent? And will Allah forgive a sin that its committer persists in by praying and increasing good deeds?

Viewing and keeping naked pictures of women is absolutely forbidden (haram). This is because it leads to letting one's gaze wander to what is unlawful and gives rise to satanic thoughts. Immediate repentance is obligatory, which means abandoning the sin, regretting it, and resolving not to return to it. One who repents from a sin is like one who has no sin. As for one who does not repent, their matter is up to Allah's will. Persistence in minor sins transforms them into major sins, but Allah does not forgive polytheism (shirk) without repentance.