Here is what our priest said in one of his homilies about why we cover images and crucifixes with purple cloth:
Since the 3rd Century, today (5th Sunday) has been known as Passion Sunday. A very powerful way this was demonstrated was the practice of covering or veiling the Crucifix and all statues and holy pictures within the Church with violet/purple cloth. This practice originated in Rome, where the images in the papal chapel were veiled immediately after the deacon proclaimed the traditional Gospel passage, which concluded with the words, “Jesus hid Himself and went out of the temple.” (Jn 8:59) So the focus from today until Easter has traditionally been on the Sacred Passion of our Lord! On Good Friday, the violet cloth is removed from the Crucifix so that the faithful may venerate it while the rest of the sacred images are again seen on Easter. Certainly, you could cover the sacred images in your homes with purple cloth as a stark reminder to focus on Lord’s Passion!
"The veiling also symbolizes the fact that Christ's Divinity was hidden at the time of His Passion and death, the very essence of Passiontide." Fish Eaters