Specification of an Image field attached to forum topic node type?
have you created one image field that is shared between all of your content types, or a different image field for each content type? if it is the latter, this sounds like it is likely a permissions problem where one of the image fields is restricted to admin users only.

i'm not sure about D7, but there's a module in D6 called "content permissions" that seems like you should enable it but you really should avoid it if you can. There's probably an equivalent in D7 - the module denies access to all new fields for all user roles by default, and you have to explicitly give permissions back.

alternatively, it could be a caching issue. Drupal caching is much less aggressive for admins than for anonymous users so if you've made a few changes to your views that admins can see but anonymous can't, try clearing your site's cache.