Yes, networks are not able to be viewed by public (guest) users as they cannot have joined a network. What you are asking for doesn't make sense to me though. It cancels the privacy completely. Because, that content that you are hiding for logged in members per network would be available to non-logged in members. So for your scenario:

1. Sally is a member of the Cat Network. As she isn't in the Dog network, when she is logged in, the content is filtered and she sees Cat network albums.

2. Sally logs out and now she can see the Dog network content.

With the above, the content isn't hidden from Sally at all. If she wants to see content she's not allowed to see, all she has to do is log out. With that in mind, there's no reason to use Networks as they are meant to give more privacy options. 

Also, if your Dog members prefer that only Dog owners see Dog albums, by allowing public to view the Dog network, they aren't getting their private content private and they leave the site due to privacy concerns.

The only solution I see to your concern is to:

1. Allow admin to choose to let users select whether non-network members can view a public thumbnail of their album.

2. Allow users to set a publicly viewed thumbnail.

3. Allow users to choose who can view that thumbnail - guests, site members, friends, network members.

Needs a lot more thought than the above but you get the idea perhaps from what I put.