custom user page image field crashes when uploading, removing image
In the menu entry for your custom page, if you have not already done so, you need to add the file path and file to reference the user module files, like so...

$items['my/custom/page'] = array(
    'title' =&gt; 'My Title',
    'page callback' =&gt; 'my_page_function',
    'access callback' =&gt; TRUE,
    'file path' =&gt; drupal_get_path('module', 'user'),
    'file' =&gt; 'user.pages.inc', 
  );


I find it's necessary to include the reference in the function, as you have already done, and also include it in the menu entry.  Don't forget to clear the Drupal cache after updating the menu entry.