I don't have RMXP installed anymore, so I can't provide screenshots (or the exact name of the event codes to call), but I can give you the general gist.
This is a very simple event to do. You make an event activated by the action button without a graphic. You place this event on top of whatever you want to be a dresser. And you give it the following event codes:
Show Choices (the one that shows 1-4 options in a dialog box)
Choice 1 (Black Dress)
Set actor graphic (choose whatever clothes you set this choice to be)
Choice 2 (Blue Dress)
Set actor graphic (choose whatever clothes you set this choice to be)
Choice 3 (Red Dress)
Set actor graphic (choose whatever clothes you set this choice to be)
Choice 4 (More options...)
Do another "show choices".
If you want an example of a game that does this, check out The Mirror Lied. In the main character's bedroom, you'll see a dresser that does this.