Well it's not just a random guess on how many dots to add...
In a directory path on most file systems, "../" means simply go up one directory. In your case, the last directory that the two paths had in common between the script's location and the upload directory is the "root" directory. Do you see what I mean by the last directory "in common"? The "root" directory was the last directory they shared before they split off into different directions.
So, what you need to do first in the script is to get back to that common directory, and then work from there. Since you're two directories past that directory ( "root/store/admin/" ), you need to go back two directories. So, our path so far is:
../../
Now, we're back to the "root/" directory. From here, just add on the path to the upload directory, leaving you with:
../../sections/images/store/main_pic/