Pictures (paintings, photographs, etc) are protected by copyright, for the life of the creator plus 50 years in most countries; the US and others have extended this to life plus 70 years. This means only the creator can copy, alter, distribute, or display the work, or authorize others to do so.
Copyright affects using photos from the internet in the same way it affects using physical photos: any use by anyone other than the copyright holder requires permission.
I can't take your photo from Flickr and use it on my website without your permission in the same way that I can't take a photo out of your photo album and print it in a book.
Depending on the specific text of the country's law, Photography is either lumped under "artistic works" or the even broader "literary and artistic works." Photographic works are automatically protected by copyright as soon as they are fixed in a tangible medium, which includes digital photos. Unless other arrangements were made, the photographer automatically has the exclusive right to copy, alter, distribute, or perform/display the image, or authorize others to do so, for a limited time. In most countries, the duration of protection lasts for the life of the photographer plus 50 years; the US and several other countries have extended this to life plus 70 years.
You need the permission from the owner of the copyright. This is usually the person, relative(s), business or corporation that created the image, or owns the copyright. If you copy an image of someone else's photograph, for instance, and use it as your own, without permission, you could be prosecuted for infringing the rights of the copyright-holder. In some cases, the copyright has run out, and the image is available for free use - I think it is after 70 years, but I'm not sure.
You need permission from the copyright holder or an exemption in the law.
"Copyright constraints" is just a way of saying what you want to do is limited by copyright. If you wanted to use a piece of music in a movie, but the fee the rightsholder wanted was beyond your budget, you would say you couldn't use the song due to copyright constraints.
You may use copyright protected material when you are the copyright holder, or when you have permission from the rightsholder or an exemption in the law. The most notable exemption is fair use or fair dealing, which allows certain limited unlicensed uses in situations such as education and commentary.
Copyright infringement is against international law.
You can only use content for which you are the copyright owner, is in the public domain, or you have permission from the copyright holder or an exemption in the law. But your resulting web page is automatically protected by copyright as soon as you create it.
If your use is not covered by an exemption in the law, get permission in writing from the copyright holder.
In some instances, copyright law is seen to be in opposition to freedom of speech: you can say whatever you want, but you can't say what he just said, because he owns it.The fair use clause of copyright law goes some way toward easing the tension, but uneven application of fair use has led to an environment in which a use is assumed to be infringing unless the user can prove otherwise: essentially you're guilty until proven innocent. As stated in the Techdirt article linked below,"...if we really believe in the First Amendment and that fair use is supposed to protect it from over aggressive use of copyright law to censor, then fair use should be assumed, and the legal burden for proving that a use is not fair should fall on the copyright holder."
Websites are protected by copyright, so you would need permission from the copyright holder or an exemption in the law to use someone else's web content.
Infringement is the use, without permission, of copyrighted material that does not fall under a "fair use" or other exception to copyright law,
They use the copyright law to protect the proprietor (owner) from plagiarism and copyright infringement. This is useful because the ideas of others will stay their ideas as well as entitle them to all benefits and profits.
Included in the copyright law is the notion of "fair use," which allows certain specific unlicensed uses such as this. See the link below for a good discussion of copyright and fair use in the classroom.
Using protected materials is legal if you have an exemption in the law or permission from the copyright holder.