So, your question is if you are allowed to use just any image in your articles?
No. You are not allowed to do that. Most images are under normal copyright, prohibiting exactly that. You can though ask the owner of the rights, most of the time the maker, if you are allowed to use the photo. Another option is to search on Flickr and 500px after images with an Creative Commence Licence. Depending on the CC-Licence that is added to the image, you are allowed to use it with just naming the photograph. Or you buy images on sites like GettyImages.
And if you have an image, you are allowed to use, the owner often likes to be called out in the image text.