Is It Illegal To Possess Pictures Of A Topless 16 Yr Old If You're Over 18?

Yes and does it matter if its eleagal you should be ashamed

If you get caught with them you are in trouble legally, if her parents find out they could make trouble for you both legally........the best to you

Yes it is called minor pornography

First of all, the answer will depend on where you are, and how do you get the image.
If it´s an "internet picture", downloaded from a website, there could be different cases:
-In some european countries it´s not illegal. Furthermore, they have magazines where it is normal to display girls of that age topless. Keep in mind that in some of those countries nudism of all ages is quite common also.
-There are other legislations where it may not be considered pornography (as long as she is not displaying her genitalia).
-Even more, some law systems will not prosecute you, since that would be considered a "privacy act", and as long as you don´t trade or hand over the image.

On the other hand, if you took the picture, that´s more likely to be considered a crime, regardless of the usage of the image (except in some places where, as I sated, it is not illegal).

My advice:
1-don´t get yourself in troubles. There are thousand of pictures of naked (and legal) 18yo girls.
2-If you took the image, get rid of it, and never distribute it.
3-again, don´t get yourself in troubles!

Most definately.  A 16 year old is a minor.  It can get you a lifetime of having to register as a sex offender.  Is a picture of a 16 year olds assets really worth it?

No, the legal definition of child pornography does not state that a topless only image is pornographic. Look at 553US___2008  United States v Williams- [ however] if you are concerned as to whether some over jealous law enforcement officials define  child pornography and any nudity by those under 18. Recently some district attorneys have threatened to file charges over pictures of girls in bra and panties-they were found inside their cell phones. Point is---unless you want to defend yourself and pay legal bills [ which can hit $1,500,000.00] then delete the image.