As soon as you create a painting, write an article or take a picture you have copyrights to your work. You don't have to register it, or mail it to yourself, or put a copyright symbol on your work, it just happens.
When someone uses your work without your permission (copyright infringement), they are liable for your actual damages, and in some cases your legal expenses too. Often the actual damages caused by someone using something from your website are small, much less than the cost of taking them to court. If you have registered your work with the U.S. Copyright Office you may be able to collect for your court costs too. You can often convince a copyright violator to quite violating your copyrights by having your lawyer send them a letter explaining that you are planning to sue them for actual damages of $100 plus your legal fees of $10,000 and that they will have to hire a lawer to defend them for another $10,000, or they can just remove the offending item from their website.
You can use tools like TinEye to find unauthorized copies of your images on websites, and you can use Google to search for very specific text strings taken from your website to see if anyone is copying your materials.
The legal doctrines of "fair use" (when the benefits of using the copyrighted material outweigh society's need to protect the copyrights of the owner) and "de minimis" (so unimportant that the case is a waste of the court's time) allow some flexibility in using copyrighted materials without permission, however copyright law is so unclear that you can't be certain of the outcome of any copyright case that goes to court. Works by the federal government and works that were created long ago are in the "public domain" and you can use them.
It is easy and tempting to copy beautiful pictures from some website and incorporate them into your website, but it is not legal. You don't want other people to "steal" your content, and you shouldn't steal their "content" either. There are plenty of Stock Photography sites that will license pictures for use in websites for a small fee, usually $3 - $10. Often manufacturers will provide free pictures of their products that you can use when selling their products, but ask for permission.
We assume that you have permisson to use any picture or text that you give us to put in your website. That means that either you created it, or you were granted permission by the owner of the copyrights.*
*We are not lawyers and we can't give out legal advice, so be sure to discuss your copyright concerns with your lawyer.