Fix image size jumping regression

Fixes https://github.com/vector-im/riot-web/issues/6654

src/components/views/messages/MImageBody.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2015, 2016 OpenMarket Ltd
+Copyright 2018 New Vector Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -140,7 +141,6 @@ export default class extends React.Component {
     }
 
     onImageLoad() {
-        this.fixupHeight();
         this.props.onWidgetLoad();
     }
 
@@ -208,6 +208,7 @@ export default class extends React.Component {
             }).done();
         }
         this._afterComponentDidMount();
+        this.fixupHeight();
     }
 
     // To be overridden by subclasses (e.g. MStickerBody) for further