Big, -depends, -depends, -only when needed, -from the beginning. -usually Main or Aux ...essentially what Bloodsong just said. One little digression, but a tidbit worth noting regarding the working size of your composition window: When a Background Image is imported, it is resampled to the current size of your composition window, and it is this new resolution, rather than that of the original file, that is actually used during the renders -- often resulting in lossy multiple rescaling errors. (Sheesh! I had to type that four times to get it into a sentence that sounded like it might make sense) So, to reduce the interpolation errors when importing background images, either make your composition window the same size as your final render, or the largest even multiple that will fit. It appears that once the image is imported, you can then rescale the composition window back down without further detrimental effects. or, blow all this off and just apply your background image to a square prop with the Object Color Zero Black and the Ambient Color at 100% White.