Making empty parts of aerial imagery transparent in QGIS?

First of all I would place the raster as the bottom layer, then the white cells won't hide anything. If this is not an option try the layer Properties > Transparency tab. With the plus sign you can define color transparency for a specific color.

I'm guessing the areas with no imagery all have the same NoData value?

1. right click on your layer and select "Layer Properties"
2. go to "Transparency"
3. top right "Additional No Data values"
4. enter the value of the white areas