Well, I might as well add my 2¢ to the giant pile of change.
In general, I am a very timid forum-poster, one who lurks a lot, reads most everything, seldom posts (and when I do post, it tends to be massive walls of text that few read). Something about a forum of people discussing things makes me feel uncomfortable, for some reason. I don't fill out my profile, and I seldom have a signature ever. I think that they tend to look bad, because few people spend a large amount of time working of them, especially when it comes to images. I especially hate it when they are riddled with misspellings.
My vote: Text signatures, 3-6 lines tall. Cap image height at ~60 pixels.
Really, even though I would prefer to not look at them, I would rather not disallow people the opportunity to have an image in their signature. And really, if an image is too obnoxious or irritating, then you can send a quick PM to the person to ask them if they could tone down their images a bit.
When you have a large group of people, you are going to get a weird phenomenon: people have different opinions and cultures. When you have different people with different opinions, I believe that concessions have to be made, especially from both of the extremes.
The culture of our previous board has dictated that images are not allowed. However, with this new board, we have a sudden change of culture: images are suddenly allowed. Instantly we have a schism occurring, from the side who wants to embrace the new culture, and the side that wants to embrace the old culture. Not that either culture is bad, per say, they are just wildly different. When you have a sudden culture shock, an unfortunate tendency is for the groups, who may have had no problem with each other before, to dehumanize the other group. As long as the two groups remain separate from each other, and as long as they have wildly diverging beliefs about the issue, some members of the group will never see each other's opinion as valid, or even humane.
In my eyes, an opt-in/opt-out system will just keep these two groups separate, and would perhaps lead to a subtle culture schism in the board.
So, while I would prefer not to have images, I voted for a concession. Will I ever use an image in my signature? Probably not; it's not my style really. But others will enjoy it, at the concession that they have to keep it reasonably sized.
Having an opt-out option is definitely a good idea, for technical reasons, and for people who really despise all images. However, I'm not sure that it would be the wisest course of action to make opt-out the default setting, especially when we're just trying it out for the first time. The board has been compulsorily image-free since the day it was digitally conceived, lets give it some graphical freedom for a while. If it gets to be too much, then we could always go back to the old way. If we don't try both, how will we ever know what we prefer?
EDIT: I hate it when I think of something else to include in a post about 5 seconds after I hit the submit button. Anyways, just to head off a potential argument:
And if we do find that we don't want images in out signatures after trying it out, then some of the people will complain, yes. However, the people who would complain would probably be part of the problem that would lead to disallowing images, and it wouldn't be taking away something they enjoy just to spite them, it would be taking away something to punish them, and to take away temptation. Besides, for the most part, we are a reasonable group of people, and if it came to that, then most people would understand. Most people tend to fall in the middle of any group anyways.