[Bug]: MultimodalConversableAgent iostream fails to serialize PIL.Image

### Describe the bug

Bug report from [Discord](https://discord.com/channels/1153072414184452236/1212552055642660935/1341360595739475969)

### Steps to reproduce

```python
import os

from websockets.sync.client import connect as ws_connect

import autogen
from autogen.io.websockets import IOWebsockets
from autogen.agentchat.contrib.multimodal_conversable_agent import MultimodalConversableAgent

config_list = [
    {
        "api_type": "openai",
        "model": "gpt-4o",
        "api_key": os.environ["OPENAI_API_KEY"]
    }
]


def on_connect(iostream: IOWebsockets) -> None:
    print(f" - on_connect(): Connected to client using IOWebsockets {iostream}", flush=True)

    print(" - on_connect(): Receiving message from client.", flush=True)

    # 1. Receive Initial Message
    initial_msg = iostream.input()
    print(f"{initial_msg=}")

    try:
        agent = MultimodalConversableAgent(
            name="visionAgent",
            system_message="""You are a helpful assistant capable of analysing images.""",
            llm_config={
                "config_list": config_list,
                "stream": True,
            },
        )

        user_proxy = autogen.UserProxyAgent(
            name="user_proxy",
            system_message="A proxy for the user.",
            is_termination_msg=lambda x: x.get("content", "") and x.get("content", "").rstrip().endswith("TERMINATE"),
            human_input_mode="NEVER",
            max_consecutive_auto_reply=10,
            code_execution_config=False,
        )

        print(
            f" - on_connect(): Initiating chat with agent {agent} using message '{initial_msg}'",
            flush=True,
        )
        user_proxy.initiate_chat(
            agent,
            message=initial_msg,
        )
    except Exception as e:
        print(f" - on_connect(): Exception: {e}", flush=True)
        raise e


with IOWebsockets.run_server_in_thread(on_connect=on_connect, port=8765) as uri:
    print(f" - test_setup() with websocket server running on {uri}.", flush=True)

    with ws_connect(uri) as websocket:
        print(f" - Connected to server on {uri}", flush=True)
        print(" - Sending message to server.", flush=True)

        initial_msg = """What's the breed of this dog?
<img https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0>."""
        websocket.send(initial_msg)

        while True:
            try:
                message = websocket.recv()
                message = message.decode("utf-8") if isinstance(message, bytes) else message

                print(message)

                # if "TERMINATE" in message:
                #     print()
                #     print(" - Received TERMINATE message. Exiting.", flush=True)
                #     break
            except Exception as e:
                print("Connection closed:", e, flush=True)
                break
```

```console
- test_setup() with websocket server running on ws://127.0.0.1:8765.
 - on_connect(): Connected to client using IOWebsockets <autogen.io.websockets.IOWebsockets object at 0x168611890>
 - Connected to server on ws://127.0.0.1:8765
 - on_connect(): Receiving message from client.
 - Sending message to server.
initial_msg="What's the breed of this dog?\n<img [https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0>](https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0%3E)."
 - on_connect(): Initiating chat with agent visionAgent using message 'What's the breed of this dog?
<img [https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0>](https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0%3E).'
 - on_connect(): Exception: Unable to serialize unknown type: <class 'PIL.Image.Image'>
 - IOWebsockets._handler(): Error in on_connect: Unable to serialize unknown type: <class 'PIL.Image.Image'>
Connection closed: received 1000 (OK); then sent 1000 (OK)
```

### Model Used

_No response_

### Expected Behavior

_No response_

### Screenshots and logs

_No response_

### Additional Information

_No response_

Labels: bug
State: closed