On the Content with Style blog Pascal Opitz looks at creating a custom Zend form element for uploading images.

It really is time to post something, isn't it? Here is a quick way to have an image preview inside of a form based on Zend Form. I am using the auto include mechanism for this one, utilizing the PEAR naming convention, and all my files will sit underneath a Shared folder within the library folder.

Their custom element is created in a two pieces - the form element itself and a custom view helper to define how the form element is displayed. An example of its use is also included.