Questions about the new Spacer v3.1

i notice the Spacer has been updated and can now act as a seperator... what does this mean exactly?

Separators have some extra features:
1) TLB order icons by any criteria between separators. So is the Spacer work as separator it stay in place when you apply auto-arrange. Else TLB think this is the usual button and apply the sorting order to the Spacer too
2) Separators don't selected by keyboard navigation

so if i put a picture in it and align it left, and then fill in an amount of pixels to be the 'Size of empty space (pixels):

The size is ignored when you select the picture. As I see now the alignment field have a bit incorrect words. It must contain values Right (Bottom) and Left (Top)

Alignment used to align the image if the width or the height of the buttons is greater then the selected image. So inside the menus and vertical toolbars alignment can be left, right or center. On the horizontal toolbars it can be top, bottom or center.

then i think the size field should go disabled when i add a picture to show it will now have no effect like the previous version did.

Yes you are right.