Sink or Swim, Fin Fan 4 Life
The sizes vary based on the source of the image. Unless you have a photo editing program to resize them, most people just copy the link from where they found the image and post it. There are forums out there that have it set to where it automatically resize images. But I am not sure how that is done. You will have to ask Bill about that one.
As for multiple pictures. That sounds like people quoting other posts with a picture in it. And as for user sigs, that really is up to the user to when they change it. If you don't like looking at the same sigs over and over again you can shut those off in your user settings so you don't have to see them anymore.