Not sure if it’s what you want, but there is a photo calendar hack in the Wiki – http://wiki.wordpress.org/index.php – or searching the forum may help too.
You could specify the image as a backgound of the caption (where it displays the month) something like:
table#wp-calendar caption
{
background-image: url(http://calendarimage);
background-repeat:no-repeat;
background-position:top right;
}
Obviously depending on how big your calendar and image may be you may have to add some padding, change the size of the caption, reposition the image but it would work.
