Usually they're hosted separately (some vps and tumblr in this case). So it's just a matter of the blog using the default mapping for it's logo to the blog site

Usually they're hosted separately (some vps and tumblr in this case). So it's just a matter of the blog using the default mapping for it's logo to the blog site. The blog is generally completely unaware it's psuedo-participating in a larger site configuration. Good from a security perspective, a bit of a hassle for admin'ing (hence this issue).