We generate them here. They were supposed to add SWF thumbnails to 4chan years ago but didn't for some reason. Desuwa has them in his catalog so I don't know why not on 4chan. It could have something to do with security concerns.
It's not exactly safe to run/thumbnail every uploaded SWF, since malicious files are uploaded all the time. It's not big risk for us since, I've put security measures in place for that here. But still.
Even perfectly innocuous looking flashes can be dangerous or just silently collect information and send it out. Like this >>>/f/3080395
and this >>>/f/3093802
are examples of flashes that call home and send information.