How to implement CSS-defined images from bootstrap site in wordpress theme

Multiple solutions for defining the images ->

Post Thumbnails

While creating a new post, you can set the Featured Image which will automatically set the post image.

Since your image is full width, you need to set the size of the thumbnail using: https://codex.wordpress.org/Function_Reference/the_post_thumbnail

Media Upload

  1. Go to Media -> Add New. Upload your image and after upload is complete, click on Edit.

  2. On the right hand side, copy the File URL in the save metabox and replace that URL in your CSS. You can view the list of all images in Media -> Library later on.

This solution is for editing the code directly.

Advanced Custom Fields

If there is a need to allow an image uploader directly replace this image, you need to use Advanced Custom Fields Image upload field and refer that field in your header.php file for the src attribute.