Exactly what it says. The only limits in place are the number of screens that can be paired, and the size of uploaded images (25mb or less).
Any device playing one of your playlists is counted as a 'screen'. Since everything is powered by your web browser, that means that a 4x4, 16 monitor matrix display will count as a single 'screen'.
Anything with a web browser! We recommend something that will run the chrome browser, but any browser will work. We even support Google Chromecast playback, but we don't recommend it if your playlists consist largely of videos.
To cast your playlist, launch one of your screens on a desktop in the Chrome browser, then click the 'cast' button and select the screen you want to cast to. When your screen is casting, you'll get a message telling you that you can close that tab and it will continue playing.
Nope, not really. If you'd like to design your own slides, you can set a slide to just be an image that you upload.