Conventional long picture hook pins, do not work with walls made of concrete and hard bricks.
Use these moulded hooks instead, with short hardened steel pins pre-loaded. Hammer them into the hardest surfaces. They can usually be easily removed, leaving minimal marking.
Width of hook at top: 20mm
Width of hook at bottom: 8mm
Overall height of hook: 40mm
Use cord, not wire. Do not use these hooks for very heavy or valuable pictures.
The prices shown below are for packs of 10 hooks.