Adding thumbnails independent of images in blog posts

Is it possible to add thumbnails to blog post listings independent of
images in a blog post

set the ‘featured image’ when writing/editing your post; and use the_post_thumbnail() within the loop, in the template.

e.g. if there is no image in the blog post, can I add a thumbnail
image?

dependent on a post image, you can check (in your template, in the loop) if the post contains an image, and then output the post thumbnail, for instance with:

if( !strpos(' '.$post->post_content, '<img ') ) the_post_thumbnail();

details depend on your design.