set default width/height if it's a local image

lib/ImageViewer.js

@@ -42,13 +42,19 @@ export class ImageViewer extends Component {
     // zoomed image
     zoomedImageStyle: Image.propTypes.style,
     zoomedImageProps: PropTypes.arrayOf(PropTypes.object),
+
+    // required if it's a local image
+    imageWidth: PropTypes.number,
+    imageHeight: PropTypes.number,
     // callbacks
     onMove: PropTypes.func,
     onPress: PropTypes.func,
   };
 
   static defaultProps = {
-    doubleTapEnabled: true
+    doubleTapEnabled: true,
+    imageWidth: width,
+    imageHeight: height / 2
   };
 
   constructor(props, context) {
@@ -66,8 +72,8 @@ export class ImageViewer extends Component {
     this.layoutListener = null;
 
     this._imageSize = {
-      width: null,
-      height: null
+      width: typeof props.source !== 'object' ? props.imageWidth : null,
+      height: typeof props.source !== 'object' ? props.imageHeight : null,
     };
 
     this._layoutX = 0;
@@ -106,11 +112,10 @@ export class ImageViewer extends Component {
 
     if (typeof source === 'object' && typeof source.uri === 'string') {
       Image.prefetch(this.props.source);
+      Image.getSize(this.props.source, (width, height) => {
+        this._imageSize = { width, height };
+      });
     }
-
-    Image.getSize(this.props.source, (width, height) => {
-      this._imageSize = { width, height };
-    });
   }
   componentWillUnmount() {
     this.state.layout.x.removeAllListeners();