Is an obscene phone conversation considered adultery (zina), and how can one atone for this sin and for sending immodest pictures?

What you did, such as sending pictures and being in seclusion, and so on, is a temptation from Satan and a reprehensible evil, a pretext for corruption and a means to fornication. Although it is not actual fornication necessitating a حد (prescribed punishment), it is metaphorically called fornication. You have done well by regretting what you did, so make it a sincere repentance, fulfilling the conditions of repentance. Sending pictures is a reprehensible act from which one must repent. If you can convince this young man to delete the pictures, that would be a good thing. We advise you to strive for marriage and search for a righteous man, and to beware of everything that incites lust, and to be keen on the means of righteousness and staying away from the traps of Satan.