Quoting the forum rules:
- Large images in signatures are also very annoying to most users. Keeping them at or below 600x120 is recommended.
I know this policy hasn't been rigidly enforced, although we do take action when someone is blatently taking the piss (e.g. huge images or 4MB animated GIFs).
Are you willing to name and shame the worst offenders? (PM if you like)