If you took the top paragraph of the care bear sig, and shrunk the care bear image down to match that height, your sig would probably be fine.
In general, sigs are advertising banners - they are advertising you, the stuff you write, the place you work, etc. It should be compact and tasteful (whether I've succeeded, below, is difficult to say).
Also, for those who don't like the pictures, you can turn them off. Go to your user profile, click on "Edit Options" at the top, go 2/3rds of the way down the page and look for:
Show user's signatures in their posts? yes no
Show user's avatar in their posts? yes no
Show image attachments and code in Posts? yes no
[i][size=1]Selecting no will show a hyperlink instead.[/size][/i]
The first option is the one I would go with, since it is a rare day indeed that a sig holds important information.