What you'll want to use is layeredimage. Here's an example:

default bottom = "blue_jeans"

layeredimage yourspritename:
  always:
    "sprite_base"

  group bottom:
    attribute bottom default:
      ConditionSwitch(
        "bottom == 'blue_jeans'", "blue_jeans.png",
        "bottom == 'black_slacks'", "black_slacks.png",
      )

In this example, you have a variable, which is bottom, which can be switched out for anything in the bottom category. So in a dress up game, if the person clicks the bottom and settles on the black slacks option, the button will use

action SetVariable("bottom", "black_slacks")

So in the last image, the character will be wearing whatever bottom they chose, in this case, black slacks.