I have plaster walls. The two best options are:
1. Only put the picture where there is a stud. Use the stud.
2. Go to home depot and buy the expanding molly bolts (depending on the weight of what you're weighing there are different sizes.) You have to be careful with the molly's, tho, because you don't want to crush the plaster, u simply want the bolt to expand. Some of the bolts are called mollys some are called differnt things for each style, but if there is no stud, thats what you can do.