I don't understand what kind of 'credit' you wanted? Glowing compliments? Did you even have permission to post pictures of her daughters on FB?
I see nothing wrong with what the mom did. You tagged them thereby giving them permission to use the pictures. Unless you are a professional photographer who took professional photos and didn't get paid, I don't think you have much complaint.
tagging is not giving permission to use the pictures. She doesn't have to be a "professional" to get credit for taking the picture.
Tagging is not sharing? I disagree. Once you post a picture on FB, its really difficult to control how people use it. The mom used a pic, that she was tagged on, as her cover photo. I see nothing wrong with it.
What kind of credit did OP want? Money? Recognition? Seems a little self involved if she's annoyed simply because she didn't get any praise for a good picture. Someone out there posted "good picture." That's praise. Why isn't that enough?
Because it's a legal thing, not a social thing. That is why I didn't get into it any further. I'm objecting to the idea that because you post something online and let people know about it, you are automatically giving up control of that picture. That's not true. Tagging someone in a photo does not give someone the "right" to use that photo in any manner that they chose. You (general) may not care about how your photos are used when you upload them to a website, however, that doesn't mean that someone has the "right" to use them in any way they want to.
OP has already said that she doesn't want to go through the hassle of contacting people to ask them to give her credit, nor does she want to watermark them. However, because of this, she can't expect people to automatically know that she wants credit for taking the pictures that they are sharing with others.
Also, just because I don't post any pictures without a watermark doesn't mean that I think people who do are wrong. I am fully aware of the fact that most people assume that anything they see online is free to use in any way that they want or that there is no need to give credit. I also know that there are a lot of people who are happy to share their pictures with friends, family, and acquaintances without any thought of credit or legalities. But if more people were aware of the fact that you should attribute credit to pictures you share, maybe the OP wouldn't have posted.