How do I make a loop of decals?

You pretty much have the right idea, however to make this a bit cleaner, we can use for loops and tables inside the while loop instead.

If you don't know what tables or for loops are, I highly suggest you check out these sources on them before I continue:

Tables - http://wiki.roblox.com/index.php?title=Table#string.format

For loops - http://wiki.roblox.com/index.php?title=Loops#string.format

Scenario

Now because you didn't really specify what you want to do with these decals, I'm just going to give an example of creating one on a part, and changing it's asset id every tenth of a second.

local Part = Instance.new("Part",workspace) -- New part in the workspace
local Decal = Instance.new("Decal",Part) -- New decal in the part

-- Table that holds all of our decal images
local Assets = {}

-- While loop to keep the cycle repeating itself
while true do

    -- As DevSean pointed out, this would probably be a nice little feature to add if your asset table is empty.
    if #Assets > 0 then

        -- For loop to iterate through the Assets table, with "i" representing the keys, and "v" representing the value.
        for i,v in pairs(Assets) do

            Decal.Texture = v -- Assign the texture of the decal to the new "v" asset

            wait(0.1) -- Wait a tenth of a second
        end

    -- Else, meaning is the asset table is empty, then...
    else

        -- And perhaps we could add this wait here if your asset table is empty, maybe in case you want to insert some assets into your table at some other place in your script.
        wait()

    end
end

Honestly, I'd explain a lot more, but if I where to go in depth about every detail I wrote above, I'd run out of characters to answer this question. So if you have any questions specifically about what I wrote above, just leave a comment or message me on my ROBLOX account and I'll get to it ASAP.