You can insert a spacer plugin with your own picture just above or under a normal separator... I even think that's how you made your screenshot...
What's the difference with what you're asking for? ???
My own wish for a future spacer plugin feature is the possibility to add a text label (in custom font/color of course)... I know, it is possible to create a text label in some simple graphics program like Paint and export it as png, but it's a tedious job if you want to change it...
Windows 7, IE 9.0, Shell 6.0.6000.16386, TLB 4.2.3 beta