If you make it 100% transparent, you won't see it at all [:))]
I you really want to make it with Sweet Home 3D without editing OBJ or MTL files, you can use an imported half transparent texture.
Editing the MTL file with a text editor and changing the d value of the material you want to be transparent (0.0 for fully transparent and 1.0 for fully opaque) is probably easier though, and may lead to more consistent results with the photo creation tool.

If you're not trying to modify an existing 3D model, using the transparent boxes Hans pointed to will be very helpful too.