In The Bag Trophy/Achievement
You will be able to increase your inventory after finding a backpack shortly after getting all 3 dog heads and ending up outside. If you played the 'Mia' tape upstairs in the house you will know where this location is however if you didn't you must walk through the house that Marge is in and make your way to the docks to find the Backpack.
Here is a video guide if you are stuck!