Some notes: In the very early days I had a folder for 2chan pictures. I used the same folder for w2ch images and initially for 4chan stuff. Due to chan filenames it's completely impossible to know which site they came from. I only started to separate out the two after about December 2003. No one really expected 4chan to survive after it died like the fifth time…
I think these were largely rejects. The banners were created by contest/random selection in the early days. Not every single one of them made it in. Some you see here may actually be in the rotation, in which case, sorry. Otherwise, I saved them for posterity. Let me know. Send me screencaps and money to prove your point. I saved all of these on 9 May 2004 and I'm not willing to do the unix date thing yet, so deal.
This one I'm sure is still in rotation. Interesting thing about this is that I have the source and it's dated 27 September 2003 with standard filename (1059511194834.jpg). Being in English, it makes you wonder if it wasn't posted on w2ch.
I also have the image that's the source for the Chidori one which is dated twelve hours from the filename of the banner ad itself. Given how slow the site was back then, it had to be the same image…
Some of these reference commercial anime and I'm sorry about that. If you hold the rights or whatever and take offense, I'll take them down.