Of course. If you add an image to the post you are preparing, which contains a video or an audio file and therefore you want it to go to the Video or Audio&Podcast wall respectively, the image will serve as the cover. If, on the other hand, you do not add an image, the video will display, as a preview, the first frame, while the audio will display a generic icon of the application.
This image below shows the difference between adding an image as a cover, and not adding it, to an audio file.
Remember, if you add more than one image, the post will go to the Blog&News wall.
You can click HERE to know more about the walls of content.