diff --git a/includes/providers/class-open-router-image-provider.php b/includes/providers/class-open-router-image-provider.php
index cc11203..c3acb06 100644
--- a/includes/providers/class-open-router-image-provider.php
+++ b/includes/providers/class-open-router-image-provider.php
@@ -222,11 +222,24 @@ class PRAutoBlogger_OpenRouter_Image_Provider implements PRAutoBlogger_Image_Pro
 	 * @param string $model  Resolved model id.
 	 * @return array<string, mixed>
 	 */
+	/**
+	 * Supported aspect ratios for OpenRouter image models (Gemini, GPT-5).
+	 * Arbitrary ratios like 150:79 are rejected — must snap to one of these.
+	 *
+	 * @var array<int, array{w: int, h: int, ratio: float}>
+	 */
+	private const STANDARD_ASPECTS = [
+		[ 'w' => 1,  'h' => 1,  'ratio' => 1.0 ],
+		[ 'w' => 3,  'h' => 2,  'ratio' => 1.5 ],
+		[ 'w' => 2,  'h' => 3,  'ratio' => 0.6667 ],
+		[ 'w' => 4,  'h' => 3,  'ratio' => 1.3333 ],
+		[ 'w' => 3,  'h' => 4,  'ratio' => 0.75 ],
+		[ 'w' => 16, 'h' => 9,  'ratio' => 1.7778 ],
+		[ 'w' => 9,  'h' => 16, 'ratio' => 0.5625 ],
+	];
+
 	private function build_request_body( string $prompt, int $width, int $height, string $model ): array {
-		$gcd    = $this->support->gcd( $width, $height );
-		$ar_w   = (int) ( $width / $gcd );
-		$ar_h   = (int) ( $height / $gcd );
-		$aspect = $ar_w . ':' . $ar_h;
+		$aspect = $this->snap_aspect_ratio( $width, $height );
 
 		return [
 			'model'      => $model,
@@ -245,6 +258,30 @@ class PRAutoBlogger_OpenRouter_Image_Provider implements PRAutoBlogger_Image_Pro
 		];
 	}
 
+	/**
+	 * Snap pixel dimensions to the nearest supported aspect ratio string.
+	 * 1200×632 → ratio 1.8987 → nearest is 16:9 (1.7778).
+	 *
+	 * @param int $width  Width in pixels.
+	 * @param int $height Height in pixels.
+	 * @return string Aspect ratio string like "16:9".
+	 */
+	private function snap_aspect_ratio( int $width, int $height ): string {
+		$target  = $height > 0 ? (float) $width / (float) $height : 1.0;
+		$best    = self::STANDARD_ASPECTS[0];
+		$best_d  = abs( $target - $best['ratio'] );
+
+		foreach ( self::STANDARD_ASPECTS as $candidate ) {
+			$d = abs( $target - $candidate['ratio'] );
+			if ( $d < $best_d ) {
+				$best   = $candidate;
+				$best_d = $d;
+			}
+		}
+
+		return $best['w'] . ':' . $best['h'];
+	}
+
 	/**
 	 * Enforce the interface's dimension contract.
 	 *