This Masked Liquib scene illustrates an interesting technique for combining two different images.  First, an image that included the stop sign and the church was loaded.  That was converted into a Stencil by using the Create Mask screen, and that Stencil was set as the current Mask.  The same image was also set as the Mask Background Image.  The result is that portions of the image, based on its light/dark areas, became static -- in this case the stop sign, church, etc.  Then, another image was loaded and manipulated with Frost, Drop, Flow, Bubble and Aura Effects, while the frozen Mask elements appear to float above the evolving scene.  (A much simpler way to accomplish something quite similar is to use one of the Set Mask Stencil/Background options, which instantly and simultaneously convert the current display to a Mask and also capture the same display as the Background Image.)
Liquib Stop Mask