Symptoms

When a form has a view, containing an image attachment control which is populated from a DB record on form load. The form is set to "Anonymous". When the form loads, a login prompt opens.
When removing the image control, the form works. When cancelling the login prompt and the form loads as expected, but the image will be missing. 
 

Diagnoses

Expected behaviour as per http://help.k2.com/onlinehelp/K2smartforms/UserGuide/4.6.10/default.htm_SF-ViewControls-ImageAttachmentWithProgress.html?TocPath=K2%20Designer|Controls|_____15
 

Resolution

Users will be unable to upload or download an attachment from the Image Attachment control when Anonymous access is enabled by default as per the article above.
There is an option to configure the form to enable Image Attachments, but this should be carefully considered, since exposes the form to a risk from a security perspective.