docs: update Presenton config with Google Gemini Flash for images

- Configure IMAGE_PROVIDER=gemini_flash with Google API key
- Update .env.example with all available IMAGE_PROVIDER options
- Document that Presenton does NOT support Azure DALL-E
- Add notes about alternative image providers (pexels, pixabay, dall-e-3)

Note: Gemini Flash configured but image generation not working yet
May require additional troubleshooting or Presenton version update

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

opt/02-core/presenton/.env.example

@@ -13,14 +13,24 @@ CUSTOM_MODEL=gpt-5
 TOOL_CALLS=true
 
 # ============================================
-# IMAGE GENERATION - Azure DALL-E 3
+# IMAGE GENERATION
 # ============================================
-IMAGE_PROVIDER=dall-e-3
-OPENAI_API_KEY=your-azure-dalle3-api-key-here
+# Available IMAGE_PROVIDER options: pexels, dall-e-3, gemini_flash, pixabay, none
+# Note: Presenton does NOT support Azure DALL-E (only standard OpenAI DALL-E)
 
-# Azure DALL-E 3 Base URL (if needed)
-# Note: Presenton may need custom configuration for Azure DALL-E endpoint
-# Standard endpoint: https://vsam-mhmldl4f-swedencentral.cognitiveservices.azure.com/openai/deployments/dall-e-3
+IMAGE_PROVIDER=gemini_flash
+GOOGLE_API_KEY=your-google-api-key-here
+
+# Alternative: Stock images from Pexels
+# IMAGE_PROVIDER=pexels
+# PEXELS_API_KEY=your-pexels-api-key
+
+# Alternative: OpenAI DALL-E 3 (requires standard OpenAI key, not Azure)
+# IMAGE_PROVIDER=dall-e-3
+# OPENAI_API_KEY=your-openai-api-key
+
+# Alternative: No image generation
+# IMAGE_PROVIDER=none
 
 # ============================================
 # ADVANCED FEATURES