This is hard to explain.

I need to be able to show an image in a sidebar, which links to an outside site when clicked. I need to show a different image each day of the month, linking to a different site for each image.

At the moment, I'm using an iframe which shows a particular date of the calendar, and in that calendar are 30 or 31 different images as the data for each date. It's a cumbersome solution, and I'd rather use a Wordpress method instead.

You could use a PHP widget

PHP Code:

<?php
$current_month = date('n');
switch ($current_month) {
   case 1: echo '<a href="link1"><img src="/path/to/image1.jpg" /></a>'; break;
   case 2: echo '<a href="link2"><img src="/path/to/image2.jpg" /></a>'; break;
   case 3: echo '<a href="link3"><img src="/path/to/image3.jpg" /></a>'; break;
   case 4: echo '<a href="link4"><img src="/path/to/image4.jpg" /></a>'; break;
   case 5: echo '<a href="link5"><img src="/path/to/image5.jpg" /></a>'; break;
   case 6: echo '<a href="link6"><img src="/path/to/image6.jpg" /></a>'; break;
   case 7: echo '<a href="link7"><img src="/path/to/image7.jpg" /></a>'; break;
   case 8: echo '<a href="link8"><img src="/path/to/image8.jpg" /></a>'; break;
   case 9: echo '<a href="link9"><img src="/path/to/image9.jpg" /></a>'; break;
   case 10: echo '<a href="link10"><img src="/path/to/image10.jpg" /></a>'; break;
   case 11: echo '<a href="link11"><img src="/path/to/image11.jpg" /></a>'; break;
   case 12: echo '<a href="link12"><img src="/path/to/image12.jpg" /></a>'; 
}
?>