Well, the player only has 4 slots. Not "slots" per say, but slots that can be mounted by anything anywhere.
So you have your normal slots like $backSlot and $hipSlot but then you have slots 0 1 2 and 3 which is what slot you mount them on.
0 is used for weapons
1 is used for the second hand in akimbo weapons
2 is used for the pain emitter
and 3 is used for anything you want.
the correct syntax to mount an object to slot 0 with mountImage is %obj.mountImage(image, 0);
You can only have one image per slot for a grand total of 4 images mounted at once.
Be warned, if a player takes damage, and your armor is mounted to slot 2, the game will mount the pain image to slot 2, and will dismount your armor image.
If your armor is mounted to slot 1? An akimbo weapon will dismount that.
If you mount your armor to slot 3, because you can only have 1 image mounted to slot 3 at a time, that's only 1 allowed mounted image without default game functions overriding your armor slot placement. Not to mention 3rd party add-ons also mounting stuff to slot 3.
I suppose you could also package the pain function to not mount the pain image while the player has the armor equipped, but I'm not sure if you could even do that. You might have to package the damage function itself.
Also, you can't have more than 1 image mounted on the same slot, IE you can't have 2 armor pieces on $hipSlot
TL;DR: Blockland's 4 image limit at a time is loving stupid.