Most likely cause: Your image's resolution (3840x2160) is much more than the TV's display resolution (1280x720). Your WebOS Browser is downscaling the image to fit the screen, resulting in a worse image.
Make the image smaller: Create a version of the image that matches the TV's resolution (1280x720). This will reduce file size and eliminate the need for downscaling.
Try different filetypes: Try converting the image to WebP or AVIF and see if your result improves
Check TV settings: Make sure there aren't any settings messing with your image.
If these solutions don't work, you'll probably to accept the fact that that image on your webpage will look worse on your LG TV.