Images not showing when not logged in

I'm trying to create a "Photo Type" which displays a thumbnail image 150x150 as the teaser and then when clicked it displays a larger 500x500 version, which when clicked displays the full-size photo (as described in http://www.drupaltherapy.com/imagecache).

I've managed to get it to work, when I'm logged in as one user "admin" (with admin rights), but when I am not logged in, or when I log in as another user "test01" (also with admin rights), the thumbnail pictures are not displayed. I cannot for the life of me see why.

Any ideas why this might be?

Something else which I notice is that although my image field is visible to the "test01" user when I look at the "Content Types", the extra field does not appear when I go to "Create Content" as that user.

Any help/guesses would be appreciated! Thanks.