These tough plastic wall hangers are fixed to hard brick or concrete interior walls by strong pre-seated steel pins which you hammer into the wall. The hangers are good only for hard walls; like ordinary nails or screws they are not suitable for plasterboard where you'll need to use cavity fixings.
The fitting to hard walls is very strong and secure, especially the largest size, but there is a limit to the weight of picture frame they will take. The hanging weight is not specified by the manufacturer but you'll know soon enough if the picture frame exceeds the capacity of the fixing, as within days - or sometimes weeks - the strain will lead to the picture falling, Hardwall hanger and all (we had one crash to the floor at 3am with an ear-shattering racket, smashing the picture-frame glass in the process).
However for small/medium weight picture frames they're excellent: very quick and easy to fit, and so long as the picture isn't too big and heavy they last for years. A big advantage they have is when removed from the wall - one yank with ordinary pliers will do it - they leave only a small group of pin-sized holes, easy to paint over with one brush-stroke.
Apart from the hanging weight of the picture frame there is one other caution: be careful when hammering in the pins, as if you accidentally strike the hanging-hook part with the hammer, you'll break it.
Despite these minor cautions these are an excellent solution to quick picture-hanging requiring no special skills, and cheap too.