Colin Wilson wrote:

Or use very thin cord.

I suppose fishing line is about as close as you can get to invisible if
you're suspending from above - and it is quite fine relative to the
breaking strength...

The hardest thing with that would likely be how to fine-tune the drops
to the pics hang properly level, but it could probably be done quite
well by simply wrapping a few turns around a matchstick behind the
picture...

fine clear fishing line or invisible sewing thread could be used with a
Victorian style piture rail, or a simple rectangular section wood rail
higher up. Staple into it on the top side and those staples arent seen,
and nor is the nylon.

NT