May be a couple of different issues going on for you here.
First, "Play on Roku" is basically the default media player and also a protocol for the Roku OS. It is not really a channel/app, per se so you will not find in the Roku Channel Store. When dealing with casting, the Roku mobile app will use the Play on Roku player/protocol.
With the introduction of Roku OS 11.5, 3rd party casting apps can no longer use the Play on Roku protocol (they are blocked so you will likely get a flash of Play on Roku and then nothing). Any casting app will need to support the "Roku Channel" casting protocol. The ones I know that have been updated to use the Roku Channel protocol include Web Video Caster, Tubio, Castify, and XCast. All these will allow you to cast both web content to your Roku devices as well as local storage from media servers and phone files (pictures, videos, etc.).
As for the Roku mobile app, you should be able to cast any files (video, audio, and photo) from the device that the Roku mobile app is installed on to your Roku devices. Now, I have had what you described when using the Roku mobile app and using the Media tab for casting local storage to my Roku devices or RokuTV (Play on Roku will show - sometimes it will actually get stuck there, othertimes gets stuck on black screen.) Basically, once you select the photo/media file, the Play on Roku should fade into the photo/media file. When this does not happen (gets stuck between Play on Roku and black screen), I usually just close the mobile app, restart the phone/tablet, and then relaunch the app.
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