I know how to develop apps for Android but never tried developing games, and I want to start soon. For developing games we need sounds and high quality images (say a background, characters, logos, etc.), so how to create these? For this should we have a separate team? Or a developer can create these using any tools?
There are multiple options for where the art comes from. It's perfectly possible for the developers to create the art themselves, or have a separate team make the art. Probably the easiest way to get started on this is just finding free art assets and using those as place holders or even the final art if it's good enough.
I suggest you check out these links to help learn more about the process: