May I ask what size your signature is? because it doesn't seem bigger than my last one and a png version of it fit the limit with room to spare. And I guess I'm the only one that uses png from the start. In my case it has nothing to do with increasing quality, it's retaining it. If you save a jpeg as a png they will probably be exactly the same, if not having the jpg a little higher quality.
And besides, this is a thread for opinions. And, sorry Mario, but no amount of +1s are going to change my opinion. And my opinion is that a file size restriction would have been a much better way to go. And if you don't think there is any change in quality between large png's and small jpg's, then that's all the more reason why a size restriction would be better. It would be almost impossible to notice!