This has been bugging me for a while.
Is there any way (even via scripting) to make a picture shown through the show picture command not through? Not in the sense that the player cannot pass through the picture, but the player will instead be on top of the picture instead of going under it.

Only via scripting. Picture objects by default are on a higher (z) layer than sprites. You would need a new picture class that shows on a lower layer.

However, if the image is small enough you could put it in a tileset or character set & use an event to display it.

What, exactly are you trying to do?