Tools > Options > Preview

Preview panel background color - This is the color you see behind the preview images in the main window. By default it's black, but it can be changed to any color you wish.

Preview quality - VideoMach can render a temporary low-resolution media clip to show roughly how will the output look like. This is especially important when synchronizing video, picture filters and audio. Preview resolution specifies the maximum widht and height, while the actual size depends on the source video proportions.

Folder for temporary files - Is the place for storing temporary preview movies. By default VideoMach uses the Windows Temp folder. Since temporary files can be pretty large, make sure you choose a disk with enough space. Preview movies are automatically deleted upon normal program termination.