This picture is a mishmash of several Treehouse of Terror segments scanned from The Simpson's 2007 Mini-Calendar.
Let's see if I can identify the characters...
Krusty is the doll from Season III's Clown Without Pity a nice spoof of the Twilight Zone episode Living Doll. From the same episode, I believe Sideshow Mel, Principal Skinner & Edna are zombies from Dial Z, For Zombie a spoof of several Romero "Dead" movies, Pet Semetary, et al. Best line: Bart exclaiming, "Dad, you killed the zombie Flanders!" only for Homer to reply, "He was a zombie?"
From Season IV has Bart as a bat and Grampa vamping it from Bart Simpsons' Dracula with a surprisingly absent Monty Burns from the picture. My favourite bit is where Homer tried to stake Burns in the heart...only to repeatedly stab him in the crotch. Devil Flanders and Donut-headed Homer are from The Devil and Homer Simpson, a spoof of the several movie versions of the Steven Vincent Benet's Faustian short story The Devil and Daniel Webster. Mmmmm. Forbidden donut. The Gremlin chowing down on some meat is from that episode's Terror at 5 1/2 Feet recreating William Shatner's & John Lithgow's Zoned character, Bob Wilson.
From Season VI, we get Groundskeeper Willie who is of course, Freddieesque from Nightmare On Evergreen Terrace.
Patty & Selma are bewitching from VIII's Easy Bake Coven. Jasper as a dog from Season II's Bart the Monster is a tribute to Invasion of the Body Snatchers.
I'm not sure about The Hell's Kitchen monster or Lisa as a snail, may be the Raven episode from Season I...I'm not sure. Maggie looks pretty generic and the UFO could be from any Kang & Kodos appearance. Anyone know for sure? Please correct any errors in the comment's section.