I just ran a google image search for small spoiler icons. Given the size limitations, I couldn't find anything that made it clear what the button was. The closest I found was a few buttons with an "S" in them. Here's one for you:
"S" icon image
Odd. I have books written by the owner of that website the icon was found on with the google search function.
There is also this one, hard to read but it might fit better with the coloring of the site. It says "Spoiler" right on it, but again, it is so small it is hard to read. I resized it to a 21 x 15 and attached it for you. That seems to be about as close as I could get the height without it becoming unreadable with the width at the maximum.