We use media player to display price boards in jpeg format for our store. We have the files built in google slides & we just download as jpeg & copy to the USB drive. Sometimes after making changes to the slides & we plug it back into the tv the images show to be there but they are black. It doesn't do this every time & really don't know how I've got it fixed in the past. Any suggestions?
I would start by trying to restart the Roku from Settings->System or Settings->System->Power or just unplug the power for a few seconds and plug it back in.