https://github.com/ColorlibHQ/fancybox-for-wordpress/issues/86

Author: andyluak

assets/css/fancybox.css

@@ -901,3 +901,7 @@ body.compensate-for-scrollbar {
         max-width: calc(100% - 10px);
     }
 }
+
+.fancybox-content {
+    height: auto!important;
+}

fancybox.php

@@ -292,7 +292,7 @@ function mfbfw_init() {
 <style type="text/css">
 	.fancybox-slide--image .fancybox-content{background-color: ' . $mfbfw['paddingColor'] . '}'.$hideCaption.'
 	' . ( isset( $mfbfw['overlayShow'] ) ? '' : 'div.fancybox-bg{background:transparent !important;}' ) . '
-	' . 'img.fancybox-image{border-width:' . $mfbfw['padding'] . 'px;border-color:' . $mfbfw['paddingColor'] . ';border-style:solid;height:auto;}' . '
+	' . 'img.fancybox-image{border-width:' . $mfbfw['padding'] . 'px;border-color:' . $mfbfw['paddingColor'] . ';border-style:solid;}' . '
 	' . ( isset( $mfbfw['overlayColor'] ) && $mfbfw['overlayColor'] ? 'div.fancybox-bg{background-color:' . hexTorgba( $mfbfw['overlayColor'], $mfbfw['overlayOpacity'] ) . ';opacity:1 !important;}' : '' ) . ( isset( $mfbfw['paddingColor'] ) && $mfbfw['paddingColor'] ? 'div.fancybox-content{border-color:' . $mfbfw['paddingColor'] . '}' : '' ) . '
 	' . ( isset( $mfbfw['paddingColor'] ) && $mfbfw['paddingColor'] && $mfbfw['titlePosition'] == 'inside' ? 'div#fancybox-title{background-color:' . $mfbfw['paddingColor'] . '}' : '' ) . '
 	div.fancybox-content{background-color:' . $mfbfw['paddingColor'] . ( isset( $mfbfw['border'] ) && $mfbfw['border'] ? ';border:1px solid ' . $mfbfw['borderColor'] : '' ) . '}
@@ -303,7 +303,7 @@ function mfbfw_init() {
 	' . ( isset( $mfbfw['titleShow'] ) ? 'div.fancybox-caption p.caption-title{display:inline-block}' : 'div.fancybox-custom-caption p.caption-title{display:none}div.fancybox-caption{display:none;}' ) . '
 	' . ( isset( $mfbfw['titleSize'] ) ? 'div.fancybox-caption p.caption-title{font-size:' . $mfbfw['titleSize'] . 'px}' : 'div.fancybox-caption p.caption-title{font-size:14px}' ) . '
 	' . ( isset( $mfbfw['titleColor'] ) && $mfbfw['titlePosition'] == 'inside' ? 'div.fancybox-caption p.caption-title{color:' . $mfbfw['titleColor'] . '}' : 'div.fancybox-caption p.caption-title{color:#fff}' ) . '
-	' . ( isset( $mfbfw['titlePosition'] ) ? 'div.fancybox-caption {color:' . $mfbfw['titleColor'] . '}' : 'div.fancybox-caption p.caption-title{color:#333333}' ) . $captionPosition  .$close_button.' 
+	' . ( isset( $mfbfw['titlePosition'] ) ? 'div.fancybox-caption {color:' . $mfbfw['titleColor'] . '}' : 'div.fancybox-caption p.caption-title{color:#333333}' ) . $captionPosition  .$close_button.'
 </style>';
 ?>
 <script type="text/javascript">