diff --git a/system/src/Grav/Common/Media/Traits/ImageMediaTrait.php b/system/src/Grav/Common/Media/Traits/ImageMediaTrait.php
index 59da4cc50a..4ea0ebf98a 100644
--- a/system/src/Grav/Common/Media/Traits/ImageMediaTrait.php
+++ b/system/src/Grav/Common/Media/Traits/ImageMediaTrait.php
@@ -183,6 +183,9 @@ public function derivatives($min_width, $max_width = 2500, $step = 200)
                 $derivative->resize($width, $height);
                 $derivative->set('width', $width);
                 $derivative->set('height', $height);
+                if ($base->format !== 'guess') {
+                    $derivative->format($base->format);
+                }
 
                 $this->addAlternative($ratio, $derivative);
             }