AIf you are using a Drop In image, first verify Image Folders is set to "Not Used" and then set Image Source to "Use Drop In". Be sure to go to Stack's blue edit mode and within the Layout panel, uncheck Constrain Width & Constrain Height. This will allow the original image size to be used & thus Image ReSize will be able to use the full size available to it.
If you are using a Resources or Warehouse image, first set Image Source to "Use Resources/Warehouse". Then set Image Folders to either "Use Resources" or "Use Warehouse folder". A corresponding image input will appear. If using Resources, enter a relative path to the image - Ex. Image in root of Resources: myimage.jpg or if it's in a folder w/in Resources: myfolder/myimage.jpg If using Warehouse, you can enter either a relative or absolute path to the image.