For the post I simply reused sample rss cover but scaled it down to 1024 x 1024 pixels. Original is 1400 x 1400 pixels and 483 KB - by Apple spec must be 1400 x 1400 to 3000 x 3000 pixels and I opted to keep size under 500 KB (to avoid any issues). I know it is on smaller side, but it is just a sample cover - should be replaced during real deployment, but otherwise it should work properly as a demo.