Make Bean Layer: Scrape the mixture onto a 12- to 14-in. round serving plate and smooth into a 10-in. circle with a spatula. Arrange tomato slices around edge of bean layer to make a border.
Make Middle Layer: Drain salsa in a strainer to get rid of excess juice. Spread over bean mixture, leaving some of the bean layer showing around the edge. Sprinkle cheese over salsa, leaving some of the salsa layer showing.
Make Guacamole Layer: Halve avocados; remove seeds and spoon flesh into a medium bowl. Add lime juice, onion, jalapeños and salt, and mash until fairly smooth. Spoon over cheese layer and spread into an even layer.
Make Web: Place sour cream in a ziptop bag; seal bag and snip a small tip off one corner. Pipe a spiderweb design over guacamole and onto the plate. Decorate with toy spider. Serve with tortilla chips.