Everything that is printed, published, or on the Web, including text, video, pictures, photos, music, etc. is copyrighted and may not be used except by permission of the copyright holder. It does not have to have a copyright symbol or say that it is copyrighted.
Exception: a work is in the public domain (not under copyright) or available for use without permission if it fulfills one of these criteria:
Even if a work is in the public domain, it is plagiarism to copy and paste text or even to closely paraphrase text by changing words and putting it into your article. Your article must be original. The best way to make it original is to read multiple sources and then put them away and write in your own words what you understand. Using only one or two sources can be a temptation to plagiarism.
The Wikpiedia Training Modules modules explain more about plagiarism and especially about where to find images that you can use for your Wikipedia articles.