Save image to node via form API
It's been a while since I've done this but this link should be a good start. File Load

Assuming $form_state['storage']['image'] is the image's fid, what you may want to do is. 

$image = $form_state['storage']['image'];
$file = file_load($image);

// Change status to permanent.
$file-&gt;status = FILE_STATUS_PERMANENT;

// Save.
file_save($file);

// add image back
$node-&gt;field_image['und'][0] = $image;


Depending on what your doing you may want to add the file to the usage table. Details here.
File Usage Add