Flickr has reasonably fine-grained privacy controls. There are a lot of people who do post the occasional NSFW picture right in with their regular photostream.
You can mark it private to yourself only, or family, or contacts, and anyone not in those groups will never see it. You can and should also mark it as having a different "safety" level - by default people browse Flickr in a "safe" mode, unless they change their account settings. Such people, even if you've authorized them, will be prompted to click through. This is my favorite setting, as a clicked-through image has an emergency kitten escape button.
Finally there are "passes" for certain photos that can override any other security setting. So it could be private to you, but you can give someone a pass (a special URL) to see it. You can revoke the passes later if you want.
You can mark it private to yourself only, or family, or contacts, and anyone not in those groups will never see it. You can and should also mark it as having a different "safety" level - by default people browse Flickr in a "safe" mode, unless they change their account settings. Such people, even if you've authorized them, will be prompted to click through. This is my favorite setting, as a clicked-through image has an emergency kitten escape button.
Finally there are "passes" for certain photos that can override any other security setting. So it could be private to you, but you can give someone a pass (a special URL) to see it. You can revoke the passes later if you want.