Actually, here is the code for an image picker field (should be in settings.php) :
Clicking inside the field opens a new win to choose an image ...
PHP Code:
<p><label for="sitelogo" >Logo</label>
<input id="sitelogo" name="sitelogo" class="text" value="<?php get_theme_setting('sitelogo'); ?>" type="url" onClick='window.open("../admin/filebrowser.php?CKEditorFuncNum=1&returnid=sitelogo&type=images","mywindow","width=600,height=500")' /></p>
Clicking inside the field opens a new win to choose an image ...