When it comes to images, most WordPress themes nowadays make use of featured images. In case you haven’t uploaded featured images for your posts yet, many WordPress themes usually display image placeholders instead of images on archives or within custom widgets.
If you would like to replace the image placeholders with some real images and in case you don’t have suitable images at hand, you could use free images which have been published under a free license for use on your WordPress website: 10 Image Sources For Your WordPress Website Or Blog.