How to extract files from system.img:

What you will need:

1) ext4 unpacker

2) ext2explore

PART1

Extract both zip files.

Open ext4 unpacker folder and double click on ext4_unpacker.exe

Click as shown below

Select system.img

Click as shown below

Write any name as shown below

Click on Save and wait till below green processing bar disappears

Close ext4 unpacker

PART2

Now open ext2explore and double click on ext2explore.exe

Click on file then open image

Select the file you have created in PART1.

Click on open

Click as below

Click as below 

Select the folder in which you want to extract your files,click on OK and wait process to complete.

Close ext2explore

Now you have successfully extracted files from system.img in your selected folder.