OK - I need some good ideas for a flexible picture
hanging system...

Requirements
============
I want to be able to hang some pictures on a wall
(plastered breeze block), but to have the ability
to remove the picture and the hanging mechanism and
have a nice blank wall left behind. Then some time
later to hang some pictures again, but in different
positions...

for example see

www.gillandroy.com/diy/pichang

Current thoughts
================
It seems to me that I'm not going to manage to get
a completely blank wall left behind when I remove
a pictures. So the question is how to minimise the
"damage"...

One plan is to embed some sort of screw threads into
the wall in a grid across the wall, that can be
blanked off when not in use. This will enable me
to attach/detach whatever hanging mechanism I choose.
When detatched I'm left with a hole in the wall that
needs to be covered in some fashion. Perhaps something
along the lines of the mushroom thingies like these...

www.gillandroy.com/diy/mushrooms

Not sure exactly how to implement/procure all
this - ho hum...

Any ideas/problems/guidance/thoughts would be
appreciated...

Thanks,
Roy