Cache problem

hmm... I'm expericing the same issue.

Captcha module needs to define a hook_init() so it can clear the user/register cache in those cases. Code would be as follows:

/**
 * Implementation of hook_init().
 *
 * We place this hook in it's own module because hook_init() forces the whole module to load
 * this can be a performance hit depending on the size of the module, so we place in it's own
 * module for super fast loading
 */
function mc_image_captcha_init() {
  global $base_root;
  
  // note arg(0) is not yet available yet, so we have to use $_GET
  if ($_GET['q'] == 'user/register') {
    // clear the cache for this page since we are using image captcha and if a user reloads
    // it should show a new image, not a cached one
    db_query("DELETE FROM {cache} WHERE cid = '%s'", $base_root . request_uri());
    
    // since we cleared the cache for this page
    ob_start();
  }
}

here is my workaround to the problem. first I used the cache exclusion hack mentioned here: http://drupal.org/node/23797#comment-75367

then I made the following modifications to textimage.module:

change

function _textimage_image() {
  //if we don't have GD2 functions, we can't generate the image
  if (!function_exists('imagecreatetruecolor')) return;

  // Set headers

to

function _textimage_image() {
  //if we don't have GD2 functions, we can't generate the image
  if (!function_exists('imagecreatetruecolor')) return;

  $GLOBALS['do_not_cache'] = 1;
  // Set headers

change

function textimage_captchachallenge(&$form) {
  $form['captcha_response'] = array (
    '#type' => 'textfield',
    '#title' => t('Captcha Validation'),
    '#default_value' => '',
    '#required' => TRUE,
    '#description' => t('Please type in the letters/numbers that are shown in the image above.'),
    '#prefix' => '<div class="textimage-challenge"><img src="' . url('_textimage/image/' . time()) . '" '.$imageSize[3].' alt="'.t('Captcha Image: you will need to recognize the text in it.').'"/></div>',
  );

  return $form;
}

to

function textimage_captchachallenge(&$form) {
  $form['captcha_response'] = array (
    '#type' => 'textfield',
    '#title' => t('Captcha Validation'),
    '#default_value' => '',
    '#required' => TRUE,
    '#description' => t('Please type in the letters/numbers that are shown in the image above.'),
    '#prefix' => '<div class="textimage-challenge"><img src="' . url('_textimage/image') . '" '.$imageSize[3].' alt="'.t('Captcha Image: you will need to recognize the text in it.').'"/></div>',
  );

  return $form;
}

basically, the workaround uses a fixed url on all the pages, but it makes the image dynamic. not a pretty fix, since you need to hack the drupal core, but it works great. if someone comes up with a better solution, I would love to hear it.

The fix posted at http://drupal.org/node/23797#comment-157047 and http://drupal.org/node/23797#comment-163493 fixes this on 4.7.3