You can replace images in the 751-1000 range for BGOs, blocks and NPCs (as these won't be used by the devs). To make the image appear correctly in PGE, you will need to make a background-n.ini/block-n.ini/npc-n.ini files. You will need to include some information I.E for block-751.ini:
image = "block-751.png"
name = "New block"
group = "Special"
category = "New Blocks"
You can change the properties of BGOs by creating a background-n.txt file. To change how blocks function, you will need to do that in a luna.lua file. To give NPCs AI, you will need to create an npc-n.lua file and to change some other properties an npc-n.txt file.
The handbook gives some infromation about the .ini, background-n.txt, npc-n.lua and npc-n.txt files. Enjl's Lua tutorial videos includes videos about making NPCs as well. As for blocks you should check newblocks.lua in the blocks folder in the scripts folder to see how certain fuctionalities are given.
SMM2 ID: JJF-P7G-FSF