After Effects creating video with black background yet I have a transparent images

Check this to learn more about the alpha channel and how to export your final render with the alpha channel https://www.schoolofmotion.com/blog/how-to-export-transparent-background-after-effects 

When you render you need to choose the Lossless With Alpha preset in the Render Cue or dig through the presets in the Media Encoder to produce a video with an alpha channel. All media players will show black where there is transparency. Only a few codecs support alpha channels. None of the typical delivery options like H.264 MP4 support alpha channels. The only way to use the alpha channel is to import the rendered video into a program like After Effects or Premiere Pro that supports and will use the alpha channel in video.