Seeing another woman's private area is not technically prohibited and happens regularly in mikvaot where the "mikve lady" will carefully inspect a woman who wants to immerse (source: my wife's experience across many mikvaot).

But if done in a different context that might lead to physical desire and actions, one should remember that lesbian practices are forbidden (see SA EH 20:2 and Rambam Issurei Biah 21:8).

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.

However, one should remember that lesbian practices are forbidden (see SA EH 20:2 and Rambam Issurei Biah 21:8) so any activity that might lead one in this direction would be proscribed.