I have Imported a DEM file. no problem..
I have imported my Aerials (sid). PROBLEM.
They come in where they are suppose to except that they clip to the DEM file.
I have unchecked the clip option and i have modified my model space setting to "Use Entire Model"
I even tried drawing a boundary.
Add another surface outside the DEM you already have. It can just be something flat you made in Civil 3D if that's what you're looking for.