Download Backdrop Pictures

You're going to find this is another reason to use Google Photos, instead, where the original image can remain untouched. From what I've seen, the images uploaded to Roku's streams repository are "optimized" for TV display, meaning they're normalized to avoid/minimize any real-time processing when actually being displayed.

Here's an example comparison of a file uploaded versus the image later downloaded from the Roku photo stream...

So not even "Full HD."

And that's for a 16:9 aspect ratio image. Any image with a narrower aspect will have black pillarbox bars added to the left and right to retain the original image's aspect ratio when displayed to a 16:9 display.