Its the way that the browser locate where is the image in your computer
For example if your image is not in the same folder with the html file that you want to use and let's say its in another folder named "image" (that folder should be inside the main folder with the html file)
You type <img src="image/nameofimage.formatofimage" />
and you can use the "." Before image if the image is in another place in your computer (out of the main folder)
I hope that helped.
For local assuming u are using localhost
Depending on the text editor of you put the quote it will show a popup box to locate the image file. i use dreamweaver
Generally , if you are creating code in the sololearn or any other app, it can't access images on the phone.. you have to upload to a webhost e.g like Dropbox or Google drive and use its link
But if you are creating a code in a local note editor app , then it may detect the image locally...