According to RMXP's scripts, $game_map.fog.z is 3000 on @viewport1, and $game_pictures all use @viewport2 which the z value is 2000 meaning pictures should already be under the fog. Also, taking a look at Sprite_Picture, those sprites z value is @picture.number, so... your best bet would be to experiment with the $game_map.fog.z value.
Sorry if I'm rusty but also you could try experimenting with the .z settings of $game_map's @viewport1, @viewport2 and @viewport3 and see what you get.
Basically, z value determines how sprites 'n stuff are layered. Think of it in terms of map layers/priorities, something with a low Z value is going to be below something with a higher Z value, but when it comes to viewports this could vary. Sorry I couldn't help you more than that, good luck with it.