Whether or not they send/give you any, is it an offense to ask or pressure someone under 18 for nude photos?
If you end up possessing what would be considered child pornography, even without nudity, that's a crime. Prop 83 violators face up to 1 year in county jail or even more time in state prison, a fine up to $2500 and mandatory registration as a sex offender for life.
Just asking a minor for nude photos would be solicitinga monor for a sexual purpose, which is a federal crime. A "sexual purpose" is defined as having the intent to commit a sexual activity with that minor. Contacting a minor becomes a violation of federal law when the nature of the conversation turns sexually explicit, and for prosecution, physical contact with the minor isn't required. The mere sexually explicit communication or forwarding of sexually explicit materials to the minor is enough for a conviction under federal law. 18 U.S.C. Section 2422 provides punishment for “[w]hoever, using the mail or any facility or means of interstate or foreign commerce…knowingly persuades, induces, entices, or coerces any individual who has not attained the age of 18 years, to engage in prostitution or any sexual activity for which any person can be charged with a criminal offense, or attempts to do so.” Violators face large fines and/or imprisonment for a minimum of 10 years.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.