How do I make some of the decals at a different WatTime speed?

To do this you need to use a dictionary. Dictionaries are essentially tables with keys and values, and rather than indexing the table like table[2] you'd index it by the key, table['Key'].

So, lets make a dictionary with keys of the mainslide IDs, and values of the waitTime.

local bin = script.Parent
local decal = bin.Decal
--The numbers(5.5) are how long to wait for each slide.
local MainSlides = {
["http://www.roblox.com/asset/?id=198632478"] = 5.5
["http://www.roblox.com/asset/?id=175271294"] = 5.5,
["http://www.roblox.com/asset/?id=157119414"] = 5.5, 
["http://www.roblox.com/asset/?id=177042272"] = 5.5,
["http://www.roblox.com/asset/?id=154539172"] = 5.5, 
}

function runTable(tab)
	for i,v in pairs(tab) do
		decal.Texture = i
		wait(v)
	end
end

while wait() do
	runTable(MainSlides)
end