On my LG G2 (Verizon model), the TWRP backup is located in root/data/media/0/TWRP/BACKUPS/ (folder with a bunch of letters and numbers / folder with date and numbers, etc. this is your backup)
Now this is using TWRP version 2.8.6.1
You won't be able to see it by hooking up to your computer. You will need to use a root explorer file manager and then copy that folder (one with the date) to your SD card partition (I put mine in the downloads folder to find it easier) then hook up to your computer and copy it somewhere safe.
