LG Photo Gallery app - rotate my photos?

Hi, we've plugged a USB drive into the port on our LG OLED 55C8 and the Gallery app finds it, so we can use our own photos in the app. But some of the images are rotated 90 degrees, and while the app lets a fellow rotate the image while it is selected, once it is back in the slideshow, it displays sideways again. There does not seem to be a "save" feature to use after rotation. So what's my procedure? Open the images on a computer in something like Photoshop, rotate, then save again? Thanks!