To use an image from the file system, you basically have to inline the image. To do this you have to convert it to Base64 representation, which is an encoding of the image as a long string.
There is an online tool that makes it easy. You drag your image from your file system in the tool and it will convert is to Base64. You would click the “copy image” button to put that on the clipboard and then paste that between the parenthesis instead of a URL.
This is a link to the tool:
The syntax for the link (make sure there are no spaces between the brackets and parenthesis):
Note: since you are inlining images in your checklist if you put a lot of them in your checklist size can get very large. Make sure to first resize the image locally to an acceptable size and use an image compression like JPG for the smallest size before encoding.