design: Add 0004-multimodal-i2t proposal

[sangminwoo]

Description

Add design doc for multimodal image-to-text evaluation support in strands-evals SDK.

Introduces MultimodalOutputEvaluator extending OutputEvaluator to enable MLLM-as-a-Judge evaluation for image/document-to-text tasks. The evaluator constructs multimodal prompts using strands SDK ContentBlock format and supports both reference-based and reference-free evaluation across four dimensions: Overall Quality (P0), Correctness (P0), Faithfulness (P1), and Instruction Following (P1).

Key design decisions:

• Extends OutputEvaluator to reuse rubric/model/system_prompt management
• Built-in rubric templates + convenience subclasses per dimension
• InputT=dict carries {"image": ImageData, "instruction": str}
• ImageData supports file paths, base64, data URLs, bytes, PIL Images with JSON-safe serialization

Related Issues

• strands-agents/evals Issue #128

Type of Change

• New content

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using npm run dev
• Links in the documentation are valid and working

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[afarntrog]

I think we can probably support remote images in this:

# Define cases with image data in input dict
cases = [Case[dict, str](
    input={"image": ImageData(source="chart.png"), "instruction": "What is the revenue trend?"},
)]

[sangminwoo]

Good point. we can support HTTP URLs using urllib.request (stdlib), so no new dependency is needed. For S3 URIs, we can make boto3 an optional dependency:

• HTTP/HTTPS: auto fetched via urllib.request
• S3: auto fetched if boto3 is installed, error message otherwise

Does this makes sense to you?