docs(gemini): Update image generation docs

Author: sixlive

docs/core-concepts/image-generation.md

@@ -158,24 +158,77 @@ if ($image->hasBase64()) {
 }
 ```
 
+#### Image Editing
+
+OpenAI's `gpt-image-1` model supports editing existing images. Pass your images as the second parameter to `withPrompt()`:
+
+```php
+use Prism\Prism\ValueObjects\Media\Image;
+
+$originalImage = Image::fromLocalPath('photos/landscape.png');
+
+$response = Prism::image()
+    ->using('openai', 'gpt-image-1')
+    ->withPrompt('Add a vaporwave sunset to the background', [$originalImage])
+    ->withProviderOptions([
+        'size' => '1024x1024',
+        'output_format' => 'png',
+        'quality' => 'high',
+    ])
+    ->generate();
+
+// The edited image is returned as base64
+$editedImage = $response->firstImage();
+file_put_contents('edited-landscape.png', base64_decode($editedImage->base64));
+```
+
+You can edit multiple images at once:
+
+```php
+$response = Prism::image()
+    ->using('openai', 'gpt-image-1')
+    ->withPrompt('Make the colors more vibrant', [
+        Image::fromLocalPath('photo1.png'),
+        Image::fromLocalPath('photo2.png')->as('custom-name.png'),
+    ])
+    ->generate();
+```
+
+For precise edits, use a mask to specify which areas to modify:
+
+```php
+$response = Prism::image()
+    ->using('openai', 'gpt-image-1')
+    ->withPrompt('Replace the sky with a starry night', [
+        Image::fromLocalPath('landscape.png'),
+    ])
+    ->withProviderOptions([
+        'mask' => Image::fromLocalPath('sky-mask.png'), // White areas will be edited
+        'size' => '1024x1024',
+        'output_format' => 'png',
+    ])
+    ->generate();
+```
+
+> [!NOTE]
+> The mask should be a PNG image where white pixels indicate areas to edit and transparent pixels indicate areas to preserve.
+
 ### Gemini Options
 
 Gemini offers customizations, depending on what model is selected. All Gemini image generation models return base64-encoded images only. They also return `mimeType`.
 
 ### Gemini Flash Preview Image Generation
 
-Gemini conversational image generation provides the option to edit images:
+Gemini conversational image generation provides the option to edit images by passing them as the second parameter to `withPrompt()`:
 
 ```php
-$originalImage = fopen('image/boots.png', 'r');
+use Prism\Prism\ValueObjects\Media\Image;
+
+$originalImage = Image::fromLocalPath('image/boots.png');
 
 $response = Prism::image()
     ->using(Provider::Gemini, 'gemini-2.0-flash-preview-image-generation')
-    ->withPrompt('Actually, could we make those boots red?')
-    ->withProviderOptions([
-        'image' => $originalImage,
-        'image_mime_type' => 'image/png',
-    ])
+    ->withPrompt('Actually, could we make those boots red?', [$originalImage])
     ->generate();
 ```