How to remove black rectangle background from image using CSS

you need to have background set to black.

The reason you see parts of your background as black is because the border-radius of the pictures is set to 50%:

If you want to completely remove the black background:

  • Remove the background-color: black;

If you want to (as you wrote) want to make background transparent:

  • Add background: rgba(0,0,0,0) – where 4th value controls the opacity