This is probably so basic that my issue is not specifically a Hugo issue.
How can I make images viewable on local hugo server? If I use <img src="/images/header.jpg", it will only display after uploading the site.
If you use hugo server, images will display on localhost just as they do on the live website version. In fact, the website that Hugo makes with hugo server and the one that gets build with hugo are often mirrors.
I feel like there’s some information missing from your post so that people like me can help out with your situation. Can you take a look at the requesting help suggestions? That thread contains useful information that helps us help you. 
Here’s the content which contains a link to an image: https://github.com/valzi/buildings-are-people/blob/master/content/post/Player_Investment.md
Here’s the post after it’s rendered online: https://housesarepeople.netlify.com/post/player_investment/
On my local hugo server, on my computer, no image is displayed.
Can there be a capitalisation issue here? I see that the content file uses:
<img src='/images/player_investment.jpg'>
But in the repository the player_investment.jpg file is in /static/Images/ while the /static/images/ folder has no such file in it.
Wow. That fixed it. Thanks!