This resolution setting will only affect objects with effects that require rasterization. It's a single setting that applies to ALL such objects in your file (you can't have some effects at one resolution and others at another resolution unless you expand them). All other placed images on your file that exist on the artboard will reproduce at the resolution in which they were created. Where are you seeing the gaps/white edges? That may have to do with the flattener setting you're using.

Mordy