I've made a test trying to access the image link below and it cannot be accessed from anonymous(without login):
https://www.myhostname.br/helpdesk/fron … docid=1189
Do you have created your article with image in another entity and change it to root entity in the database ?
]]>More details in forum:
http://www.glpi-project.org/forum/viewt … 45#p163745