Wat ik kon vinden in de help:
Note: To control whether or not the renderer uses the environment map's alpha channel in creating the alpha for the rendered image, choose Customize > Preferences > Rendering ,and then turn on Use Environment Alpha in the Background Antialiasing group.
If Use Environment Alpha is turned off (the default) the background receives an alpha of 0 (completely transparent). If Use Environment Alpha is turned on, the alpha of the resulting image is a combination of the scene and background image's alpha. Also, when writing TGA files with Pre-Multiplied Alpha set to off, turning on Use Environment Alpha prevents incorrect results. Note that only background images with alpha channels or black backgrounds are supported when compositing in other programs such as Photoshop.
Note: To control whether or not a background image is affected by the renderer's antialiasing filter, choose Customize > Preferences > Rendering and then turn on Filter Background in the Background Antialiasing group. Default=off.