I recently got a Roku TV. To play a slideshow from a USB stick, the picture display is zoomed to 16:9 filling up the full screen of the TV.
I am disappointed to see this issue. After investing a lot of time modifying/erasing with the EXIF information in the jpeg files, the issue is not with any metadata in the picture files. Experimenting with files from different cameras, I found the following. When a photo of 3:4 aspect ratio is played, the Roku TV displays the correct aspect ratio. It is a full sized picture with vertical bars on the sides. But when a 2:3 aspect ratio picture (mostly found in DSLR cameras) is played, the picture is blown up cutting the top and bottom. It seems that the program handles the 2:3 ratio pictures incorrectly. Third party USB media apps display the pictures correctly.
I think it should be an easy fix, if Roku takes this seriously. This is a very basic function that we expect to work well.