First off, don't rely on blog posts to tell you what you should not be doing. You need to gain an understanding of the law and the best way to do that is to read it. Coming to your own interpretation could mean less headache in the future. The first website to check out is this government website on copyrighting.
Any government website is fair game. Just look at this page and you will get a better understanding. However, make sure you read the exceptions. Certain images like logos are off-limits. Also, this should go without saying, you cannot suggest having government backing. Remember, it is possible that there are things on the website that have copyrights, if you are unclear than verification of ownership is necessary. Be wise when using state or local government sites because they may have their own copyright policies listed. Lastly, this speaks nothing about use internationally.
"How do you know that the words you speak have never been spoken?"
Most websites have a page that explain how they handle their copyrighted material; Some have contact information so that you can contact them for written consent. That being said, written consent has to be to obtained to use copyrighted material.
I have tried many times to ask for consent to post a logo on my website to show that I donate to a specific charity and each time I have been turned down. Most have explained that they get too many inquires and they cannot keep up to make sure their logo is being used properly. Either way, it is necessary to ask.
Say a photograph that you took is being used online. You never gave the person permission to use your photograph. You may want to jump the gun and send a "cease and desist" letter. Sometimes it is better to act rationally than get your emotion involved. You need to ask yourself what you intend as results. Do you want them to take the photo down? If that is all you are asking for than the best move would be to contact the person using your photo and simply ask them to take it down.
This is the same website as above. The way it works is you create a username and go through the motions of registering. It is an old school format that is not very user-friendly but it gets the job done. This is important for when someone uses your work and you want to take legal action; It helps your case. It was about $50 for one photograph. If it is important to your business to copyright your material than you should!
Now this is where legal action gets tricky. If you have your work registered and you are taking action to get it taken down, you should understand this act. To make a long story short, it states that if the work is a "transformative" use then it is not a copyright infringement. So, in some cases the person has the right to use your copyrighted material without your permission.
Whether you believe it or not, people have turned to social media as a place to interact with the world. There is no denying the power that social media has on consumers. Do you want to stay up on the times?