Hey,

To start I'm running Exchange 2010, not 2013. But am running Office 2013. (So Outlook 2013)

I was wondering if it was possible to set a picture for email addresses that are external.

Like any person email account (yahoo, Hotmail, live, gmail, etc)...

I tried googling but I came up so dry, literally couldn't find a single post about it...

Yes, you can do this, but as with all GAL entries, you need to define the object in the directory.  External recipients are frequently mail-contacts or mail-users.

See Import-RecipientDataProperty

Thanks for the reply,

but that would mean I'd have to add a user identity to the GAL, since this is an email address only I reply to I would then have to make the contact hidden in the GAL so others wouldn't be able to search it?