For one linked image per post, override that link with a permalink when on the main page

Question

Is there an existing plugin that allows a person to, in a post, put a link on an image, but when the post is shown as part of the main-page list, have that image link to the post's permalink instead.

If not, can you point me in the right direction to custom code this myself? (What files to edit, what WP functions already exist that might help me...) I have a decent grasp of functional/procedural PHP, but have only the slightest experience editing WP code.

I am using WordPress 3.9 and the theme "Twenty Ten"

Feel free to re-tag this question. I am not familiar enough with WP or WP.SE to know what tags are best.

Answer 1

My solution was to work in the twentyten theme folder, the file "loop.php"

Around line 145 I did this:

<?php $content = get_the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'twentyten' ) );
  // http://stackoverflow.com/a/6366390/1195835
  // http://stackoverflow.com/a/11886237/1195835
  // http://stackoverflow.com/a/15469353/1195835
  $doc = new DOMDocument();
  $doc->loadHTML($content);
  $finder = new DomXPath($doc);
  $classname = "post-main-image";
  $nodes = $finder->query("//*[contains(concat(' ', normalize-space(@class), ' '), ' $classname ')]");      
  foreach($nodes as $node){
   $node->setAttribute('href',get_the_permalink());
  }
  echo $doc->saveHTML();
?>

Then in the edit post screen, I simply type "post-main-image" in the "Link CSS Class" form field and update. I would expect something similar to work in other themes as well.