[Azure OpenAI] Clarify Response_Format for gpt-image-1

As discussed in MicrosoftDocs/azure-ai-docs-pr#4365, Response_Format is not supported for gpt-image-1. But current documentation is still confusing and needs more clarification for output to explain response_format parameter is not supported for GPT-image-1 which always returns base64-encoded images.

Author: nkgami

articles/ai-foundry/openai/how-to/dall-e.md

@@ -106,6 +106,25 @@ The following is a sample request body. You specify a number of options, defined
 
 ### Output
 
+#### [GPT-image-1](#tab/gpt-image-1)
+
+The response from a successful image generation API call looks like the following example. The `b64_json` field contains the output image data. 
+
+```json
+{ 
+    "created": 1698116662, 
+    "data": [ 
+        { 
+            "b64_json": "<base64 image data>"
+        }
+    ]
+} 
+```
+> [!NOTE]
+> `response_format` parameter is not supported for GPT-image-1 which always returns base64-encoded images.
+
+#### [DALL-E 3](#tab/dalle-3)
+
 The response from a successful image generation API call looks like the following example. The `url` field contains a URL where you can download the generated image. The URL stays active for 24 hours.
 
 ```json