OSc is a complex project to set up... let alone on an unfamiliar
flavour of server. I haven't used OSc for years now so these
are 'old' thoughts. SME uses ibay groups, these allow internal
connectivity separately from external ~ might be your issue.
Going external requires you to have set up the relative base
address for the site properly, it takes a bit of understanding
so you might not be calling the right folder for the images
when external. You need to check the SME logs, they will
resolve your difficulty most probably. Also use a decent
(eg Firefox) browser with developer addons that show
you what is going on under the surface (eg Firebug).