Add a /
in front of your img
folder, like this:

I just tried it on a test github pages page and it worked for me.
Source: official Jekyll documentation example: https://jekyllrb.com/docs/posts/#including-images-and-resources.
Including images and resources
At some point, you’ll want to include images, downloads, or other digital assets along with your text content. One common solution is to create a folder in the root of the project directory called something like assets, into which any images, files or other resources are placed. Then, from within any post, they can be linked to using the site’s root as the path for the asset to include. The best way to do this depends on the way your site’s (sub)domain and path are configured, but here are some simple examples in Markdown:
Including an image asset in a post:
... which is shown in the screenshot below:

Linking to a PDF for readers to download:
... you can [get the PDF](/assets/mydoc.pdf) directly.
Related
- For additional image manipulation instructions, such as align left, align center, align right, image resizing, text wrapping, etc., see my other answer here: GitHub README.md center image