How to get rid of dummy images
The themes show default / sample images for the posts that do not have their own featured images defined. You can turn these off by going to Appearance > Customize > General Settings and turning off the "Example Content" toggle.
However, please note, your blog/website will look uneven / unfinished if all the posts do not have featured images defined. So please make sure you upload a featured image for all your posts.
More information on how to upload featured images: