I have had a large painting hanging in my stairwell for a long time. Since I was taking it down I thought I'd show the method I used to hang it. I have used this on many large and heavy pictures and mirrors some well over 100 pounds. I needed something that would keep the frame level, hold it to the wall in California and be easy to hang since it's a pretty awkward area to access.
The guy at the bottom of the steps is not me he helped me set the ladder in place. The ladder is a Cosco brand really stable and really versatile if a bit heavy.
This is a product with the brand name of Zbar but there are other similar products and use the same general concept of interlocking sections - one attached to the wall and one to the frame. This comes in a light and heavy version for different weight applications. In this application I used one short piece because the painting and frame are very lightweight. Normally I'd use full length rails and on heavy or really heavy pieces I use a Zbar on the bottom of the frame and one on the top. When I do that I space them so the bottom rail to carries most of the weight. This way the frame joints are not bearing the weight.
The screws on the sides I use as stops if the frame moves sideways. Probably not necessary but I do it anyway - even here where the side walls would have stopped the frame.
The two sections about to engage. The bottom piece would be attached to the wall.
The two sections engaged.
The way I buy it it comes in a tube with four sections and runs about 30 or so but does not come with screws or anchors. I try to use enough screws so each has no more than about 5 pounds of load to carry.