Hanging pictures on a sloping plasterboard wall...?
If the frames aren't important &amp; are at least a few mm's wide, then it would be best to actually secure the pictures by drilling the frame &amp; screwing them directly to the wall. Either, directly into the structural framing or into one of many available wall anchors, I prefer one's that flare-out behind the plasterboard.

You can also accomplish the same result by using large washers on screws applied to the outside of the frames (2 on bottom &amp; 1 on each side), so there's no drilling. But, the washers can be a bit ugly. So, the better option would be a Mirror Hanger type of Wrap From Behind clip (left) or Perimeter Offset Clamp (middle) or you may have to bend &amp; possibly cut your own out of L-Brackets or Corner Braces (right) to accommodate thicker frames (Wrap From Behind application).



These will handle a very good deal of weight &amp; frequent bumping into &amp; can be padded with tape or felt to protect the frames.