diff --git a/milestones/README.md b/milestones/README.md
index 01ab8d1..3b084ef 100644
--- a/milestones/README.md
+++ b/milestones/README.md
@@ -5,10 +5,13 @@ repositories that install `docgen` and maintain their own demo bundle. The
 library no longer ships an in-repo dogfood; consumers are the integration test
 of record.
 
-**Active:** **[image-empty-bytes.md](image-empty-bytes.md)** —
-`image-generate` must not write empty PNG bytes as success.
+**Active:** **[image-generation-strings.md](image-generation-strings.md)** —
+`image_generation.model` / `size` / `quality` must be YAML strings at
+config load.
 
 **Shipped:**
+- **[image-empty-bytes.md](image-empty-bytes.md)** —
+  `image-generate` must not write empty PNG bytes as success (#103).
 - **[hint-front-matter-yaml.md](hint-front-matter-yaml.md)** —
   Invalid `hints/*.md` YAML front matter must fail `yaml-generate`, not skip
   hint `visual_map` wiring (#102).
diff --git a/milestones/image-empty-bytes.md b/milestones/image-empty-bytes.md
index 08779c4..978371a 100644
--- a/milestones/image-empty-bytes.md
+++ b/milestones/image-empty-bytes.md
@@ -1,7 +1,7 @@
 # Milestone: image-generate must not write empty assets
 
-**Status:** Active  
-**PR:** pending  
+**Status:** Shipped  
+**PR:** #103  
 **Depends on:** `milestones/hint-front-matter-yaml.md` (PR #102),
 `milestones/tts-empty-audio.md` (PR #99)
 
diff --git a/milestones/image-generation-strings.md b/milestones/image-generation-strings.md
new file mode 100644
index 0000000..dd34669
--- /dev/null
+++ b/milestones/image-generation-strings.md
@@ -0,0 +1,35 @@
+# Milestone: image_generation.model / size / quality must be strings
+
+**Status:** Active  
+**PR:** pending  
+**Depends on:** `milestones/image-empty-bytes.md` (PR #103),
+`milestones/tts-empty-audio.md` (PR #99)
+
+## Problem
+
+`tts.model` / `tts.voice` are already non-empty YAML strings at config
+load. `image_generation.model` / `size` / `quality` were not.
+
+`image_generation_config` does `defaults.update(block)`. A YAML **list**
+for `model:` (bullet list of model names) overwrote the default string.
+`generate_images_for_spec` then did `str(icfg.get("model"))`, sending
+`"['gpt-image-1']"` to the Images API.
+
+## Goal
+
+Fail closed at `Config.from_yaml`. Missing keys still use the property
+defaults (`gpt-image-1`, `1536x1024`, no quality).
+
+## Done when
+
+- [x] Present `image_generation.model` / `size` / `quality` must be
+      non-empty YAML strings.
+- [x] Tests for list values of those keys.
+- [x] `ruff check src/ tests/`
+- [x] `pytest tests/`
+- [x] `docgen benchmark` (no clock change)
+
+## Out of scope
+
+- Empty mapping `image_generation: {}` remains allowed (defaults apply).
+- `wizard.default_guidance` type gating is separate.
diff --git a/src/docgen/config.py b/src/docgen/config.py
index f85e0aa..68277ad 100644
--- a/src/docgen/config.py
+++ b/src/docgen/config.py
@@ -282,6 +282,13 @@ def __post_init__(self) -> None:
             )
         if wiz.get("llm_model") is not None:
             require_yaml_string(wiz["llm_model"], label="wizard.llm_model", source=src)
+        ig = self._block("image_generation")
+        if ig.get("model") is not None:
+            require_yaml_string(ig["model"], label="image_generation.model", source=src)
+        if ig.get("size") is not None:
+            require_yaml_string(ig["size"], label="image_generation.size", source=src)
+        if ig.get("quality") is not None:
+            require_yaml_string(ig["quality"], label="image_generation.quality", source=src)
         ocr = self._sub_block(validation, "ocr", label="validation.ocr")
         if ocr.get("error_patterns") is not None:
             string_list_block(
diff --git a/tests/test_config.py b/tests/test_config.py
index c22c1f8..36f0e20 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -465,3 +465,24 @@ def test_from_yaml_mapping_av_sync_anchor_keywords_loads(tmp_path: Path) -> None
     cfg = Config.from_yaml(p)
     rows = cfg.av_sync_config["anchor_keywords"]["01"]
     assert rows[0]["keyword"] == "Flask"
+
+
+def test_from_yaml_list_image_generation_model_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("image_generation:\n  model:\n    - gpt-image-1\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="image_generation.model must be a YAML string"):
+        Config.from_yaml(p)
+
+
+def test_from_yaml_list_image_generation_size_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("image_generation:\n  size:\n    - 1536x1024\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="image_generation.size must be a YAML string"):
+        Config.from_yaml(p)
+
+
+def test_from_yaml_list_image_generation_quality_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text("image_generation:\n  quality:\n    - high\n", encoding="utf-8")
+    with pytest.raises(ConfigError, match="image_generation.quality must be a YAML string"):
+        Config.from_yaml(p)