Apply Imagecache style to raw image url
It sounds like you should handle this in your computed field's PHP since you're already parsing out the image there.

//Assuming $entity is the parsed node in question 
preg_match_all('/&lt;img.+src=[\'"]([^\'"]+)[\'"].*&gt;/i', $entity-&gt;body[LANGUAGE_NONE][0]['value'], $matches);

//If an image match has been found, build a URI relative to public file base
//Assume image URLs are something like:
// "/sites/default/my_files/some_dir/whatever.jpg"
if(isset($matches[1][0])) {
  $public_dir = variable_get('file_public_path','sites/default/files');
  $uri = file_build_uri(str_replace("/$public_dir/","",urldecode($matches[1][0])));
  $entity_field[0]['value'] = image_style_url('thumbnail',$uri);
}