I tried to put also a keyed background, but chroma key doesn't seem to work and the PNG files without a background also don't work, it just shows a black backround. What can I do?
I would guess you are putting the png files on a higher track than the image/video you want to show through.
For transparency whether it is chroma key, alpha or an image with a transparent background it must be on a lower track than the the object you want to show through.