Symptoms

After adding an image to a form that is set for anonymous access, the image does not load and the page is prompted for authentication.
 

Diagnoses

By default, the Image Attachment Control has separate Anonymous Access settings than the rest of the form. This is due to a security vulnerability that is surfaced by the control.
 

Resolution

For information about how to change this setting, as well as the vulnerability, please see Considerations in the following documentation.

http://help.k2.com/onlinehelp/k2forsharepoint/userguide/4.7/default.htm#sf-viewcontrols-fileattachmentwithprogress.html