There is currently no Fixed Layout option (like in SimpleViewer-Pro) but thank you for your suggestion and for posting it in the Feature Requests forum thread.

If you give your gallery fixed pixel dimensions (rather than percentages), then the gallery will not dynamically resize with the shape and size of the user's browser window and its layout will not change at all. This is as close to a fixed layout as you can get (without actually having the ability to position each gallery element relative to a stage corner).