For the Normal image, the expected data type (well, kinda) is a url link to the image, not the image data directly.
Check out this documentation website that was set up a while ago:
It is a little out of date on some things, but common and unchanging methods like setObject will be correctly documented.
By and large, I would recommend against using game.sendAction directly, as it has less than helpful errors, and most of the methods call it for you with the correct data structuring, in the event of something odd happening (looking at you, collisions).
Also, if you are interested in changing multiple objects at once, setMapObjects is the go to. Just be aware of the boolean for new vs overwrites.