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 100 hooks.