I watched it pretty much all day. I wrote and posted the 'Time Travel Joe' story, and watched what the attack did to that, so I have a good idea of what happened.
Mind you that this is my best guess as to what happened, and did not in any way participate in the attack.
Around 11am cst, someone (rumour said 7chan posters) started a bot that posted the 'Nyoro~n' image with text quoted randomly out of the first post in any given thread.
For example it quoted a sentance near the bottom of the first part of the 'Time Travel Joe' story that made no sense to quote. It wasn't a catch phrase or anything. It just looked like one if you weren't paying attention to what it said.
If a thread was on the first page it got 'Nyoro~n' bombed. Posts on the 2nd and 3rd pages didn't see much traffic.
Once something was quoted it was added to a list, and the rest of the 'Nyoro~n' images would post it seemingly randomly. This was pretty obvious by about 1:00pm when it was still quoting stuff that had been posted before 10:00am.
This whole time the board was lagging badly under the flood. Rather than an invasion, it was more like a scripted DOS attack.
The mods tried to delete a bunch of the 'Nyoro~n' pngs, but they were simply coming in faster than regular posts.
Around 3:00pm or so, the mods apparently got the bot's IP addy blocked and the lag let up a little. By this time a lot of copy-cat image macros had been set up, including one of 'Cockmongler', 'That Fucking Cat', 'Happy Negro', Suiseiseiki, and a few others. I don't think they were botted, but they could have been.
So as of 6:00 or so, all 10 pages of /b/ were full of nothing but that same image macro.
My guess is that moot or one of the other admins got home from work, blocked a bunch more IPs and started manually deleting shit from /b/'s MYSQL tables.