Fines for copyright infringement vary from country to country; in the US, fees are $750-$30,000.
plagiarism
There is no preset amount that you need to change. The law is absolute that you are not allowed to create derivatives of someone elses work without permission. No matter what alterations you make all you can own the copyright to is your changes, not the underlying material.
When you copy someone elses work and you give them no credit for it.
Copyright infringement, a civil offense.
I assume you are asking the difference between plagiarism & copyright infringement. While both are essentially the use of someone elses work without permission, the most significant difference is that plagiarism also involves claiming that material as your own work.
taking credit for someone elses written work
You commit an infringement under copyright law any time you use, without permission, someone elses protected work, providing that your useage does not qualify as "fair use" or another exception under current copyright law (i.e. right of first sale, etc)
It happens all the time, and it usually isn't punished, regardless of the supposed consequences. Keep in mind that you're still not actually protected by copyright because it's not official, and if you steal someone else's work and label it with your own fake "copyright", you can still land in trouble. It's not uncommon to find a YouTube video of copyrighted work posted by someone else and with the uploader's supposed "copyright" attached to it. They're still in copyright trouble, but as you can tell, it's rarely enforced.
Unless the job is work for hire or work for employer the photographer holds the copyright.
No. Under Section §106A. (Rights of certain authors to attribution and integrity) of US Copyright Law the author has the right to credit for his/her/their work. Additionally, using someone elses work without properly citing the source would leave you open to a claim of plagiarism.
Copying and using someone elses work, claiming it your own. Piracy, also
Certain things on the Internet are copyright. This is because it is someone work/ creation and is therefore illegal to use this persons work with out acknowledgement of the creator. Anything which says copyright ( such as artists songs) are copyright.