What do you mean input your own collection? You mean Clear Logo Art (Wheel Art)? Yes you can, for BigBox which is apart of the Premium version of LaunchBox. This is geared towards Arcade Cabs and HTPC set ups being controlled by controllers or arcade controls.
No, you don't need RocketLauncher at all. Some users who came over from RL / HS still utilize RL, but it is not at all required. LaunchBox is it's own entity.
Nope, there is zero limit on the amount of games you can import. I currently have 25k entries in LaunchBox.
LaunchBox pulls metadata and media from the LaunchBox Games Database and EmuMovies. Video from EM requires EM Premium and LB Premium as well. You can supply your own media if you'd like. There are Video, Image, Music and Manual folders inside the LaunchBox folder. Inside here will be the names of the consoles when you start importing. Make sure the appropriate images go in the appropriate sub-folders (inside the system folders), refresh your LaunchBox image cache and it should attach just fine. The media needs to be named properly, which is either the name of the game inside of LaunchBox or the name of the file you are importing.
LaunchBox does not have an attract mode. Attract mode has been requested for a long time now, and is being looked at for BigBox by Jason, the LaunchBox dev.
Welcome, and if you have any more questions feel free to ask. I would also suggest that you watch the tutorials on our YouTube channel for more help with LaunchBox. You can get there by clicking the link in my signature or the tutorials button at the top of the forums!