Can you post a nude picture of yourself at 17?

There is no federal law that prohibits nudity by minors. Additionally, there is no federal case law that is directly on point that holds that minors may not produce nude images of themselves.

However, the production of "child pornography" is prohibited by US Code Title 18, Part 1, Chapter 110, Section 2256, that defines "sexually explicit conduct" to include: (v) lascivious exhibition of the genitals or pubic area of any person.

Whether or not nudity is considered "sexually explicit conduct," or is "lascivious," is questionable.

Nudity by minors for an artistic purpose, or in advertising, appears to be allowed (examples are: Brooke Shields was 15 years old when she appeared in a racy Calvin Klein jean advertisement featuring the memorable line, "Nothing comes between me and my Calvins." Shields also appeared nude at 12 years old in an Oscar-nominated movie called Pretty Baby. Similarly, 14-year-old Jodie Foster, wearing revealing clothing, played a pre-teen prostitute in Martin Scorsese's Taxi Driver.)

Most case law deals with whether or not a minor is capable of consent prior to their 18th birthday (emancipated minors or other special situations may allow consent).

State and local laws may prohibit nudity and are not considered in this answer.

US Code - http://www.law.cornell.edu/uscode/uscode18/usc_sec_18_00002256----000-.html