How to add screenshot to READMEs in github repository?
If you use Markdown (README.md):
Provided that you have the image in your repo, you can use a relative URL:
If you need to embed an image that's hosted elsewhere, you can use a full URL
GitHub recommend that you use relative links with the ?raw=true parameter to ensure forked repos point correctly.
The raw=true parameter is there in order to ensure the image you link to, will be rendered as is. That means that only the image will be linked to, not the whole GitHub interface for that respective file. See this comment for more details.
Check out an example: https://raw.github.com/altercation/solarized/master/README.md
If you use SVGs then you'll need to set the sanitize attribute to true as well: ?raw=true&sanitize=true. (Thanks @EliSherer)
Also, the documentation on relative links in README files: https://help.github.com/articles/relative-links-in-readmes
And of course the markdown docs: http://daringfireball.net/projects/markdown/syntax
Additionally, if you create a new branch screenshots to store the images you can avoid them being in the master working tree
You can then embed them using:
JUNE 3, 2020 : WORKING ANSWER-
- Upload your image to postimage.org
- Copy Github-Markdown-Url after uploading Image
- Insert in your ReadMe
I found that the path to the image in my repo did not suffice, I had to link to the image on the raw.github.com subdomain.
URL format https://raw.github.com/{USERNAME}/{REPOSITORY}/{BRANCH}/{PATH}
Markdown example 
Even though there is already an accepted answer I would like to add another way to upload images to readme on GitHub.
- You need to create issue in your repo
- Drag and drop in the comment area your image
- After link for the image is generated insert it to your readme
More details you can find here