Book about a magician's apprentice who gets stuck in a painting

Question

I am trying to remember the title of a book about a boy who learns magic.

It was so long ago all I can remember is:

• A young boy is being taught magic.
• He goes on an adventure.
• He gets stuck in a painting.
• In the painting is a powerful book of magic which he begins to study.
• He finds out how to get out of the painting.
• On his way back to his village, he somehow is travelling in a flying castle and pays back a man that he borrowed or stole a boat from during his journey.

Answer 1

This is Lawrence Watt-Evans's With a Single Spell

Tobas had been lucky to find a wizard to take him on as apprentice. But then the wizard died suddenly and unexpectedly after teaching Tobas only a solitary spell, and the youth was too old to find a new master. How could he earn a living when all the magic he could do was light fires?

A DESTINY UNFOLDING

In the Small Kingdoms there were dragons to be slain, princesses and gold to be won, magic castles, witches who knew the secret of immortality, and other treasures. But how could a wizard with a single, simple spell hope to find them and win them?

Tobas travels through a magic tapestry to a wizard's castle. Indeed, he finds a book of powerful spells from the prior owner, as well as the wizard's immortal wife. They are trapped because the return portrait is no longer accurate, but Tobas finds a way to fix that. And yes, at the end, Tobas tracks down the person whose boat he stole and gives them a replacement.