The standard way VS handles drawing a new element on top of something that doesn't have a content property, say like a image on top of an image, is that vs will craw up the hierarchy until it finds a element that does and add it to that.
For instance if i have:
grid
|_ Image (stretched to the full size of grid)
and i try to draw a new element it wont let me, I have to draw it elsewhere then drag it in, when in reality it should attach it to the grid.