All images within my articles have a link of their own: why? The article content type has a field to upload images with the article text (body and teaser). I noticed that every image I upload gets its own URL after submitting the article, e.g. http://www.example.com/imagejpg.
This is messing up my Google SEO as each image has its own URL, and it is showing up in the search index for my domain.

Is there any way to stop these images from having their own URL?
All entities in drupal have their own page, you can use the Rabbit hole module to control what should happen when an entity is being viewed at its own page.
Inside module there is a sub-modules for each entity type, you have to install rabbit_hole and rh_file.