Is the fluid that came out of me semen, requiring ghusl, or is the prayer valid? And is masturbation to relieve pain after being aroused by stimulating images forbidden?

If the fluid discharged from you is maniy (semen) — with its known characteristics — then it necessitates ghusl (ritual bath) according to the majority of scholars. There is no difference in this regard whether its discharge occurs immediately after arousal or after the cessation of pleasure. Therefore, your prayer is invalid, and you must repeat it. As for masturbation, it is forbidden according to the majority of scholars.