tec: Limit size of illustrations to 5 Mo

Too large images take a lot of place in memory and can exhaust the max
allowed memory size.

src/services/Image.php

@@ -70,7 +70,7 @@ public function generatePreviews(string $image_url): string
         models\FetchLog::log($image_url, 'image');
         try {
             $options = [
-                'max_size' => 20 * 1024 * 1024,
+                'max_size' => 5 * 1024 * 1024,
             ];
             $response = $this->http->get($image_url, [], $options);
         } catch (\SpiderBits\HttpError $e) {