Also, when Oz first became a fairyland, it harbored several witches and magicians and sorcerers and necromancers, who were scattered in various parts, but most of these had been deprived of their magic powers, and Ozma had issued a royal edict forbidding anyone in her dominions to work magic except Glinda the Good and the Wizard of Oz.

Ozma herself, being a real fairy, knew a lot of magic, but she only used it to benefit her subjects.
This little explanation will help you to understand better the story you are reaching, but most of it is already known to those who are familiar with the Oz people whose adventures they have followed in other Oz books.
Ozma and Dorothy were fast friends and were much together.

Everyone in Oz loved Dorothy almost as well as they did their lovely Ruler, for the little Kansas girl's good fortune had not spoiled her or rendered her at all vain.

She was just the same brave and true and adventurous child as before she lived in a royal palace and became the chum of the fairy Ozma.
In the room in which the two sat--which was one of Ozma's private suite of apartments--hung the famous Magic Picture.

This was the source of constant interest to little Dorothy.