Hi - can someone set my mind at rest regarding a picture I've recently
hung on a plasterboard wall?

I've used plastic plasterboard raw plugs to hang a picture (about 5
feet wide and 2 feet tall) on a plasterboard wall but am now
regretting my decision and wished I'd sourced some of the metal type
plugs instead which splay out. The picture has two mirror type fixings
top and bottom of the picture (the type which screw into the wood
frame and then require you to screw these to the wall).

Whilst the picture is quite heavy it seems stable and the screws are
certainly tight (it's been up for 2 weeks now). But I'm a little
worried that over time something may give. The screws are 1 1/2 inch
size 8 and the two fixings on the left side of the picture went into
something other than just plasterboard! I don't think it was wood, but
when drilling the holes there was certainly more resistance than with
the two right side fixings.

Any advice or suggestions would be most welcome. I don't simply want
to take the picture down and re-fix it using the metal type if the
general consensus is that it will be fine as it took my wife and I
over 1 hour to hang the picture due to its size.

Thanks for reading