Sometimes a large picture will have a section of the image that appears to be missing and is shown as grey.
With Astronomy pictures, this may be because the image is a mosaic of irregular images. Check the image on the web site to confirm this.
Occasionally, the download will have been interrupted, so the image is not complete. In this case, delete the image. Then empty the trash before double-clicking the trash icon to force the image to download again. If you do not empty the trash, Pic-a-POD will just retrieve the original image file from the trash, and so it will still be incomplete.