Yes, it's possible. But you should probably also configure your editor to allow browing that folder (there are options e.g. in JCE).

If you choose to upgrade to the Pro version, you get the added bonus that users (by default) get their own folders to manage image galleries, which makes the whole process more safe (cause if everyone has access to the same folder, accidents are bound to happen).

SIGPro uses by default a separate auto-generated folder per user. This is to make sure no single user messes with the galleries from another user. But these users can't share a folder by default. That being said, they can simply use direct plugin tags, e.g. {gallery}images/stories/something{/gallery}.

You're the first to ask really. The feedback we've received always pointed to separate user folders. If users are to share specific folders, they should probably have increased permission rights.

Noted to consider for v3.7 or v3.8 of SIGPro. I'll let you guys know...