Welcome to the Motion forum and Motion. Yes you can fill a white shape with any images.
If for example you had "White Text" and a "Background" that you wanted to appear in the white text you would need to:
Have background in bottom layer and white text in top layer, make sure they are in there own group. Apply Stencil Luma to White text. In timing pane or project pane right click on the group and choose blend mode, Normal. It is set to pass through by default so everything below those two layers disappear.
I prefer doing an Image Mask which is the same thing but you don't have the pass through problems. Be sure to click on this link to check out my Creating a Hologram video tutorial. You will learn about Image Masks in the middle of the tutorial. Best of luck and hope this helps.