Thats actually a tricky question, but here are the answers:
It is a violation of copyright law & IG terms to repost other people's content, even if you do give them credit, unless they have specifically granted you a license to do so. That being said, 99.9% of copyright holders will not object to IG reposting if you do give them credit, and therefor absolutely nothing will happen 99.9% of the time. You do still run a small risk of some people filing copyright violations even if you do give them credit.
If you are just adding your watermark to other peoples content, you are surely going to piss them off, even if you credit them, and you will super piss them off if you don't credit them, and you are going to be reported if they find out about it. If you're stealing artwork from random 16 year olds they are likely to never find out though. If you are stealing from artists with a large following, or a large corporation with an IP department, they are going to find out and file complaints. I've filed probably 100 IG reports against people for posting uncredited/re-watermarked versions of my content, and I find almost every post because my following is large enough that I'm gonna get tagged by multiple of my followers on any post that gets any traction on the explore page.
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just takes other people's Instagram posts, prints them out on a big canvas and adds a new caption for them, and sells them to retarded rich people for $100k. Courts have ruled in his favor that that falls under fair use because his captions are "transformative."
