Drag images into mask

M asks are properties of a layer, not independent entities. So no, unless you obey the rules Rick laid out there is no elegant way to just substitute images. Anything more fancy will require pre-composing the image and substituting it inside the pre-comp while masking the pre-comp in the parent comp.

Mylenium

For this to work, you must ensure the layers are the same size. You select the layer with the mask in the Timeline. Then you have to select the new layer in the Project Panel, hold down the Alt/Option key, then drag it into the Timeline. When you get over the timeline, release the mouse, and the layer source will be replaced. 

If the source footage is not the same size, your best option is to use the original layer with the mask as a track matte. If you need something other than transparency, you'll need to give us workflow and design goal details.