Commons:I found it on the Internet
A lot of people who upload to Commons have mistaken ideas about copyright. They think that just because something is found on the Internet, it has no copyright or is free to use. Sometimes the website even says the stuff is free to use, and try to upload it to Commons, and they have a very hard time understanding why it gets deleted. Wikimedia Commons does want people to upload stuff, but Commons is not like most websites. Commons wants to make sure something is truly free to use, so they have a lot of rules. If you upload something that gets deleted because of copyrights, try to understand Commons is not singling you out. It's because Commons wants to build a repository of truly free material.
Copyright is the right someone has when they create something with their mind to legally own what they have made. When they create something, called a "work", they own all the rights to that work. The rights to a work are automatic. You don't have to register your copyright or tell people it's copyrighted. All you have to do is draw something, or take a picture of something, or write something, or otherwise artistically create something, and your work has copyright. You can't lose that right just by sharing it with people on the Internet, so putting something on a website doesn't make it "public domain" and free for other people to put on their websites, no matter how easy it is to do.
The first problem most people have is they see something on a website that doesn't say it's copyrighted or says it's free for you to use, so they think it's ok. There's a few problems with that. Number one, a lot of times other people are just like you, they put stuff on their website from somewhere else. If the website you got the stuff from didn't create it in the first place, then they aren't the owners of the copyright. They can say something has no copyright, but their saying that doesn't make it true. Second, even if they don't mention copyright, it's copyrighted. Works are legally owned by the person who made them. The United States used to require people to mention a copyright, but since 1989 that is no longer true.
Another common problem people have is where the website says their stuff is "free" to use or "unrestricted", or they say it is ok for personal or non-commercial use. This is where Commons is different than most websites. Commons has a rule, called Commons:Licensing that says a work must not just be free of charge, but must also have freedom for people to use the work commercially (for a business) and to create "derivative works".
A derivative work is where you take someone else's work and change it, or derive something from it. It can be something small like drawing a moustache on a painting, or it can be something like taking a picture of a statue, or taking a photograph and drawing a picture of it. Most of the times when people tell you it's ok or free to use their work they only mean you can use the work on your own website. They usually aren't giving you commercial rights or the right to make derivative works. Since Commons requires stuff to be free for that too, it doesn't allow stuff that's just free of charge.
What's the difference? Well, think of it like free beer at a party. You get invited over and you can drink as much as you want, but there's still a lot of stuff it's not ok to do. It's usually not ok to take beer home to drink later. That would be stealing, even if you can drink as much as you want at the party. Also, some parties might not like you to get too drunk and get loud. It's also usually not ok to puke on the furniture. And of course it's illegal to drive home drunk. So even though the beer is free, there is still a lot of stuff you can't do with it! So when Commons says a work must be free, they mean "free" in a sense most people don't understand. You have to have more than just the right to partake of the work, but also many more rights.
The right to make derivative works is also something most people don't understand fully. When you copy something from a webpage, or take a picture of a statue, or scan something from a book, or make a screen capture from a TV show, you are creating a derivative work. The work is new because it is transformed into something new. But you still don't own your derivative work just because you made it. You are copying somebody else's work. And unless you had the right to make the derivative in the first place, the work is still owned by the people whose work you are copying. They still own the copyright, so you can't upload a derivative to Commons unless you have the right to.
Some countries have a rule in their law called freedom of panorama. You can click that link to see if your country is on the list. Freedom of panorama means it is ok to take a picture of a statue or sculpture in your country and put it on Commons. This is one of the few times Commons will allow a derivative work without permission from the person who owns the copyright.
So is there anything you can upload to Commons? Yes, there is. You can upload anything that is public domain or has a free license. On Flickr, if you look in the lower right hand corner of the page, it will tell you what license a photograph has. All rights reserved is what it usually says, and that's not free for Commons. If it says "Some rights reserved" that means it is under a Creative Commons license. If you hover your mouse over the words, and see where it links to, you can see which license they are using. The NC (non-commercial) and ND (no derivatives) ones are not ok for Commons, because Commons requires those rights be available. CC-BY and CC-BY-SA are the ones that are ok for Commons.
Of course, if the picture doesn't belong to the Flickr user to begin with, then their license is false. They can't take someone else's work and put a license they don't own on it.
What is public domain? Public domain means a work is old enough that the author's rights have legally expired. In European countries, this happens 70 years after the author died. Other countries might have different rules. It is important to know what the rule is for the country the work was first published in. Commons:Licensing and Wikipedia:FAQ/Copyright are good places to start for more information. If you have questions about a specific country, Village Pump or Commons:Help desk are good places to ask. Remember, something isn't public domain just because it's old, or because somebody said so. It is public domain because the rights have specifically expired by law.