Yes if:
I'm nude (Although I get naked very freely in public chat I have zero nude pictures on my profile and never post them on here, as I don't take these photos/share them freely, I don't like the idea of people taking them/sharing them)
If they're taken by a non tipper, guest/basic/someone who hadn't talked to me/tipped me or asked permission
if they're taking it for their own personal sexual gratification (without permission) or to share for others sexual gratification (always).
If it's a particularly ugly shot.
If they're planning on or do share it around forums/sites for reasons other than "go check out this awesome model!"
If it's a constant occurrence of said member.
No if:
They're doing it with my permission (for a tipper if it weren't to be shared ever I wouldn't mind nudity)
They're doing it for me, send it over to me/make sure it's ok afterwards, if I say no they then delete it (i.e. a high tip, a particularly awesome moment, a funny moment or I just look so stunning they want to take a picture to show me afterwards)
They're doing it for me for advertising reasons (They've taken an awesome screenshot of me and want to share it with the world/tell them who I am and have got my permission to post it up) - This is where if there were ANY nudity it would need to be covered up.
Generally I think ask permission first or straight after you've taken the screen shot if you just captured a moment. In real life it'd be weird/upsetting for someone to come up to me and take a photo of me without my permission or telling me what they plan on doing with the photo. If I caught them sharing/selling the photo there is legal action I could take. Although yes I am on camera so I am exposed, and taking a screen shot is so easy that most people don't even think about it, but you are taking a photo of that person. When I come on cam you get the time I am online. If you want part of me while I'm offline go private, buy my videos, buy a picture set or just enjoy the pictures I post up on twitter/on my profile. If someone if watching me for free they are already getting to watch me/enjoy the shows I do, listen to the conversations I have, I feel I give them a lot already, it annoys me that they still feel they have the right to take photos and keep them for times while i'm offline.
Basically, ask first, if you are a tipper, you probably talk to the model anyway, so ask, if you're a non tipper in a room, and guest or a basic then you shouldn't really take the shots anyway, but if you feel you shouldn't be taking them enough to not mention it to the model, then that is on their conscience. If you feel it is ok, then there is absolutely no reason why you shouldn't mention it to the model.