LG's help for "[LG webOS TV] – Photo View feature" suggests that you can copy photos (e.g. screen captures) from the LG TV to external USB, but only in certain countries. Google found the Canadian English help for this feature, which includes a screen shot of a different help system, which says
* You can copy content to a USB storage device
** This feature is available in certain countries only.
No indication what those countries are... I wonder if you change your TV setup to the right language or country it starts working suddenly. It must be some restriction imposed by TV cought holders still need about VHS taping ☹️
This is a kick in the teeth because I used the strange Focus magnification > Green button to capture screen shots of a videogame I want to sell thinking that I could copy the images... no such luck. Guess I'll have to take bad pictures of my TV screen.