It is very easy to copy and paste text and images off of other web sites. But all text and all images have a copyright attached to them unless the owner specifically places the item in the “public domain”. Owners can also allow you to use their text and images while retaining copyright themselves. They might, for example, place some images online and give you permission to use them but not distribute them; many “wallpaper” images to customize your desktop come with this license. You cannot legally place such images on your web page, because you do not own the copyright for that image. The same applies to text or images that you have in a paper book or an electronic book. If you aren’t given permission to use it, you don’t have permission to use it.
There are solutions. The most obvious is to create all of your text and images yourself. Then you own the copyright, and you can give yourself permission to use it.
You can also ask the copyright owner for permission. If they give you permission, you can use the work. If they don’t, you’ll need to find a substitute.
You can use text and images that come with permission attached. For example, if you’re looking for clip-art, go to any search engine and search for “Free Clip Art” and you’ll find lots of web pages devoted to presenting you with free clip art to use on your web pages.
And you can use text and images for which no permission is needed. If an item is in the “public domain”, no one owns it, and you can use it however you desire. Items that were published in the previous century, for example, have fallen into the public domain because copyright only lasts for about three quarters of a century (although this has recently risen). For more modern works, if it does not say public domain, it probably is not public domain.