TL;DR
Put your images in the static directory, just like below, use it in markdown like ![targets](/images/my_post_folder/my_image.png)
or ![targets](/images/my_image2.jpg)
if you don't want to build a post folder
If you search the hugo documentation, you can find Image Processing | Hugo
But! That's no a markdown way to insert an image. If you don't miss the Getting Stated, you will find the static Directory, which says can store images, that's it!
[static](https://gohugo.io/content-management/static-files/)
Stores all the static content: **images**, CSS, JavaScript, etc. When Hugo builds your site, all assets inside your static directory are copied over as-is. A good example of using the static folder is for verifying site ownership on Google Search Console, where you want Hugo to copy over a complete HTML file without modifying its content.
How to use it
Put your images in the static directory, just like below, use it in markdown like ![targets](/images/my_post_folder/my_image.png)
or ![targets](/images/my_image2.jpg)
if you don't want to build a post folder
static
└── images
├── my_post_folder
│ ├── my_image.png
└── my_image2.jpg