Now I haven’t tried this, but it does look promising. I find it hard to believe that you can get it off the wall easily afterwards (note the comment about the importance of pulling downwards in the post).
But on the other hand, if you want precise gaps between your pictures, this could definitely be the way forward. It isn’t always easy to assess the distance between the hanger and the top of the picture. Sometimes the string stretches slightly more than you expect. On more than one occasion I’ve re-jigged the arrangement rather than bother moving the nails.
So I might give hanging paintings with velcro idea a go at some point.