If you're hanging a heavy picture on a plaster wall, it's best to find a stud where you can securely anchor the wall hook. However, if you don't have a stud in the position where you want to hang your picture there are plenty of options for anchoring wall hooks into plaster walls, including hollow wall anchors or toggles that expand and spread the pressure when installed.
Light-weight pictures can hang from adhesive hooks. There are plenty of options here, including adhesive that can safely be removed from the wall without damaging the paint.
If your walls are brick, simply drill a hole with a masonry bit, hammer in a plastic wall plug or some expansion spaghetti and insert the screw.
Or, if you have timber panelling, a nail, screw or picture hook should do the job.
If you're hanging a canvas, or picture frame, that you want to sit flush against the wall, then try a flush mount wall hanging kit.