lets say following is the DIR structure of my website

Now in index.html i can simply refer images like
<img src="./images/logo.png">
but what if i want to refer the same image fron sub.html what will be the src
lets say following is the DIR structure of my website

Now in index.html i can simply refer images like
<img src="./images/logo.png">
but what if i want to refer the same image fron sub.html what will be the src
<img src="../images/logo.png">
__ ______ ________
| | |
| | |___ 3. get the filenamed "logo.png".
| |
| |___ 2. go inside "images/" subdirectory.
|
|
|____ 1. Go one level up.
Your index.html can just do src="images/logo.png" and from sub.html you would do src="../images/logo.png"
../images/logo.png will move you back one folder.
../../images/logo.png will move you back two folders.
/images/logo.png will take you back to the root folder no matter where you are/.
The relative reference would be
<img src="../images/logo.png">
If you know the location relative to the root of the server, that may be simplest approach for an app with a complex nested directory hierarchy - it would be the same from all folders.
For example, if your directory tree depicted in your question is relative to the root of the server, then index.html and sub_folder/sub.html would both use:
<img src="/images/logo.png">
If the images folder is instead in the root of an application like foo below the server root (e.g. http://www.example.com/foo), then index.html (http://www.example.com/foo/index.html) e.g and sub_folder/sub.html (http://www.example.com/foo/sub_folder/sub.html) both use:
<img src="/foo/images/logo.png">