If you just put an image into the _posts folder, I really don’t know what would happen to it, my guess is that it wouldn’t do anything, jekyll is only processing html or md files that are named with the date.
Normally I would put an image into /assets/img in the root, then reference that path in the img tag like
<img src="{{site.baseurl}}/assets/img/my-image.jpg">
Though you could put the image in other places, I have read about some people trying to keep the post together with its images, but I don’t remember how they did it. My sites are not post oriented so I just pile them all into assets/img and occasionally use sub folders in there to separate certain types of images as needed.