Transparency with a mask I would want to put a mask on a picture with the background transparent. I currently have it in the node editor. I have a mask input node and I add it to my image with a mix node. But I can only set color(like green, red, brown...) with the mix node but I would want a transparent background.
You can use the Set Alpha node found in : Add➝Converter➝Set Alpha use the mask node  output as an Alpha input for the Set Alpha node :