It's simple JC. Pictures in signatures make the pages bigger, both in terms of the size on screen and in how much has to be downloaded to view them. They make them harder to read through. It takes longer to hunt for the information in amongst the pictures as your eyes will naturally be drawn to pictures over the text. If someone posts 3 replies to a particular thread then their picture shows up three times on that thread. Do we really need to see that picture 3 times on that 1 page? NO. Avatars are fine because they are set aside from the general flow of the thread itself, but pictures and large signatures in general completely disrupt the flow of the conversation.
I know it seems petty or sad, but I write software and websites for a living. If I presented a website with pictures in between the text or content then I'd lose my job frankly. Usability and accessability is paramount. Pictures that aren't part of the content aren't.
Bringing back the old forum wont help - it certainly never supported pictures.
I hope that explains it.