When using an image instead of a pdf, it looks for an image's file name. As long as the object has a path to a file in it's expression, it won't be considered empty, so OBJECT_EMPTY won't work in that case.

There is a command called TOTALPAGES that returns the total number of pages in a pdf. So in each of your layout conditions, you could have:

Layout 1: IF (TOTALPAGES(file) >= 1, Print, Skip)
Layout 2: IF (TOTALPAGES(file) >= 2, Print, Skip)
Layout 3: IF (TOTALPAGES(file) >= 3, Print, Skip)

And so on. "file" will be replaced by the name of the file contained in your field.

Hope that helps!