I know I'm new, but I deal with signatures on a daily basis and I will offer my input anyway.

1. Do large pictures slow your downloading time?
I am on a high speed connection and most pictures are ok, but occasionally there are a few that take a while to load.

2. How many tickers should be allowed?
I think 1 tickers is plenty, especially when combined with other images and text.

3. How many lines of text should be allowed?
3-4 lines of text

4. Should we have a rule limiting bytes/pixels or just a general guideline?
I don't think a specific pixel/bytes rule will be necessary as long as everyone has an idea of what is acceptable. As a general though, wider images are easier to scroll past than taller ones.