There is no question about the fact that teacher's pics were of private nature and per se not an issue. Nevertheless, I would see a possible reason for a reprimand in reckless handling of her property, like a phone. When working with youth in a place like school, there are frew things you should guard from students with your life. They are your grade notes of students, test drafts and ay such private info relevant to students. Knowing how much such info may be stored in modern cell phones, it definitely is sloppy to let it be stolen (unless really all usually necessary precautions taken care of). For that I would consider a reprimand - but not sacking.
On the other hand, the student who stole (or took advantage of the abandoned) phone has definitely been guilty of serious misconduct (and possibly a criminal act) and least consequence would be longer period of unpaid community work or temporary expel - and possibly face criminalcharges if serious enough offence.
In a previous comment there was already a reference to Europe being different on this matter. That is somewhat true, but in Europe there also conservative corners and areas with high religious-based intolerance. In most European schools that kind of think would go out as a student prank and noone would make any serious issue out of it, the culprit student wiuld get a punishment and in a worst case (if the conditions for performing her work would have deteriorated a lot), teacher would be encouraged to seek a new position elsewhere at a date convenient to her and school would probably be supportive in helping her to find such.
Otherwise nudity in many cases is not a big issue, but in many cases teachers might be more careful to be topless on a textile beach they know their students probably would see them. On a beach with enforced nudity or a nude resort problem is not relevant as then the students encountered would be like-minded and biggest risk anyway is not bare seeing but the risk in open public spot of being photographed, taped or similar. Here also if a teacher reveals for a class that she/he considers nude swimming and sunbathing being very nice and recommending kids to enjoy it on vacation or when going for a swim a gym teacher forbids kids to go to sauna in swimwear (if he/she wouldn't, the personnel of the place would react instead) are considered nothing special and wouldn't raise eyebrows of parents.
Aki aka TravelNudie