I always was curious if a woman has to guard her eyes, like men, in tznious. I've seen a lot of bikini pics. I just wanted to know if it's an aveira to see this type of stuff.
Concur with mbloch's answer -- if one is viewing them licentiously that's a different discussion -- just wanted to bring a stronger proof. Basically, it used to be everyone of the same gender went to the neighborhood bathhouse and knew if anyone's body had serious issues. It was normal to ask your sister or mother -- hey I'm dating this girl, next time you're at the bathhouse please let me know if anything is seriously wrong with her body. And they weren't wearing bikinis! (On the other hand, I suspect they had a much broader definition of what was considered "normal" bodies.)
Rambam, Laws of Marriage 25:2--3, based on a Gemara in Kesubos:
וכן הכונס אישה סתם, ונמצא בה מום ממומי נשים שכבר ביארנום, ולא ידע הבעל במום זה, ולא שמע בו ורצה--הרי זו תצא בלא כתובה, לא עיקר ולא תוספת.
כה,ג כיצד: היה מרחץ בעיר, והיו לו קרובים--אינו יכול לומר לא ידעתי מומין אלו, ואפילו מומין שבסתר, מפני שהוא בודק בקרובותיו, וחזקה ששמע ורצה. ואם אין שם מרחץ, או שלא היו לו קרובים--טוען במומין שבסתר; ונכפה בעיתים ידועים, הרי הוא ממומי סתר. אבל במומין שבגלוי, אינו יכול לטעון, שהרי הכול רואין אותן ואומרין לו, וחזקתו ששמע ונתפייס.
One who marries a woman with no particular stipulation, and then finds that she has a physical defect as previously listed [*in 7:8, which includes several concerning breasts]; if the husband never knew of this defect, and did not accept it upon learning of it -- she exits the marriage without dowry payment.
If there was a town bathhouse and he had [female] relatives, he can't claim I didn't know she had those issues! -- even if they are in private places -- because he could ask his female relatives; we therefore assume that he already learned of them, and accepted them as-is. However if there is no town bathhouse or he has no female relatives, he can claim ignorance concerning deformities of private parts. But deformities in parts of her body that are seen by all -- he has no claim, as everyone saw them and told him; so we assume he already found out and made peace with it.
As for viewing with prurient interest -- Rav Moshe Feinstein writes that there are two reasons to prohibit a man looking at indecent material -- the anatomical effect it has on him, and that it may lead to actual prohibited relations. The former doesn't apply to women; the latter -- well it depends on the situation.
Seeing another woman's private area is not technically prohibited - and bikini pictures even less. It is hard to source a negative but extensive research didn't uncover(!) sources forbidding it.