I am quite sure you are you are looking for The magic sword from 1962.
George (Gary Lockwood) is the foster son of Sybil (Estelle Winwood), an elderly sorceress. She brought him up after his "royal parents died of the plague" in his infancy. He has fallen in love at first sight with Princess Helene (Anne Helm).
She is kidnapped by the wizard Lodac (Basil Rathbone), who brazenly informs her father that he intends to feed her to his pet dragon in seven days, revenge for the death of his sister at the same age as Helene is now: 18. George wants to go on a quest to liberate his lady love, but Sybil believes he is too young (he is 20). She tries to distract the youth by showing him a magic sword, a steed, an invulnerable suit of armour, and six magically frozen knights he will command when he turns 21. The impatient George, however, tricks Sybil and locks her in a cellar, then leaves with the magical implements and revived company of knights....
more in the wikipedia article
It has the scenes you describe and you can find the full movie on youtube.
The warriors from painting scene is at 17:20
the skeleton scene is at 38:00. (a single warrior and swamp/boiling water not quicksand)