The purpose of prepping material (copy and images) for a blog post is to have everything ready to upload (images) and copy-and-paste text when creating a new post
Create a folder on your computer for a specific project post or a specific article post (and title the folder with a short-hand post name)
Add image(s) pertaining to that specific post: One for the featured image, and others to use in the body of the post
Create a Google doc for the copy for this post and download it as a document, and save it in the same folder