Posting art/credit

Dear friends, fellow artists and watchers,
(this will be lengthy; I hope you bear with me)
yesterday I had an interesting debate about art posted in social media and giving credit. The person I talked to had a completely different approach to the subject which made me wonder: is this a generation thing, a question of individual experience or are artists in general a lot more strict about posting/sharing their artwork?

This concerns me personally and I guess that makes me pretty uptight about the topic. I have an opinion but I see where aspects might get fuzzy. For me, this is hard to judge.

Since I am asking for your spontaneous reaction, I will skip some of the many details. However, I will elaborate more clearly if necessary.

Please imagine the following situation:
a friend of yours (whom you know in real life and see daily) asked you to design art for him/her. The friend of yours is a fair bit older/younger than you, a different generation. Both of you agreed that the cooperation and all resulting artwork would be paid by your friend.
Among other things, you made some business cards. He/she liked and kept them but didn't pay them yet. He/she wanted to post them on a social media website and uploaded some pictures taken with a cellphone camera. You offered the actual files (better quality). Your friend posted them (publicly), not saying they were your artwork.
You added the information yourself, but some days after the info was deleted. You asked your friend if he/she knew anything about it, he/she said no.
In the weeks after, you kept posting the info about who made the business cards and noticing it  was deleted again, your friend stating every time that he/she didn't do it. Some time after, you found out that your friend obviously blocked you on that social media website. You confronted your friend with it, he/she saying again that he/she had nothing to do with it and it might be some technical issue...

You wait some more and ask your friend again to please give credit since you made the business cards and you can't add the info yourself (because of being blocked). He/she reacts by saying he will delete the files. You say that there is no need, he/she should just mention your name. Your friend then states that logos (the business cards were based on a logo you also made) don't feature credit/signatures. You insist on giving credit. Your friend repeats that he/she will delete the files and does not care about social media anyway. You voice your amazement. He/she then demands that you should show him a logo with credit info/artist signature in it. You state that your worry is NOT about the logo but the sharing/posting of your art material. Your friend says he/she does not want a dispute over it. And that he/she will delete the files. You ask him/her to wait and think about it. Your friend says that deleting isn't a problem and repeats that he/she does not want to fight over it. You say you don't want a fight either, just mutual understanding. You state that artists tend to give each other credit, e.g. photographers mention their models and so on. Your friend replies that yes, credit will be given on his/her website and on merchandise items, but not that social media website. You say that he/she published the business card images you created. He/she insists that he/she didn't publish your photos. You say that he/she published your work (the files are not photos, but your artwork all the same). Your friend repeats again that he/she prefers to delete the images because then things would calm down. And that he/she just published business cards, nothing else. You state that the business cards are YOUR work, period. And that he/she obviously has a much more casual attitude than you. Your friend then proceeds by asking if you ever saw a business card with credit info on it. Stating that he/she hasn't ever seen such a thing, he/she knows that of course, credit is given on websites. You say again that the point isn't the credit being directly included on the art items but on the website they were posted on! And that you as an artist don't have such a relaxed attitude. But by all means, if your friend values your work, he/she should not delete the images. Your friend says that it doesn't have anything to do with being relaxed. Then he/she asks which images, saying that he/she did NOT publish any image of yours. And that he/she will delete it later on. You reply that you both mean the same thing but keep talking past each other and that you hope that the images won't be deleted. No further reaction from your friend.

Questions:
When does artwork stop being yours, when does it start being owned by somebody else? (except when art has been paid for)
How do you feel about images of artwork - are they less creditworthy than the actual artwork?
Are social media websites less "public" than other websites?
Is it okay to not give credit to a person you plan to give credit to in the future?

...
To those who read it all and will comment - thank you!
Note that I am sure the friend didn't mean any harm. So, theft or infringement is out of question.

Looking forward to reading your thoughts about this.
~ Kat