You’ll have to replace the existing images in the theme folder.
You can have access to the theme directly from your Host Cpanel.
You need to go to public_html directly and then wp-content/themes/purea-magazine/img and then replace the image ha-image.jpg and no-image.jpg
with the same name. Also make sure the dimensions of the image matches.
Currently random image are not possible.
Note: These changes will be lost when you update the theme.