Add support for multiple DPI images, fixes #541.

docs/api/image.md

@@ -7,7 +7,7 @@ For example when creating tray or setting window's icon, you can pass image's
 file path as `String` to represent an image:
 
 ```javascript
-var appIcon = new Tray('/Users/somebody/images/icon@2x.png');
+var appIcon = new Tray('/Users/somebody/images/icon.png');
 var window = new BrowserWindow({icon: '/Users/somebody/images/window.png'});
 ```
 
@@ -23,3 +23,32 @@ file name's base name to mark it as a high resolution image.
 For example if `icon.png` is a normal image that has standard resolution, the
 `icon@2x.png` would be treated as a high resolution image that has double DPI
 dense.
+
+If you want to support displays with different DPI denses at the same time, you
+can put images with different sizes in the same folder, and use the filename
+without DPI suffixes, like this:
+
+```text
+images/
+├── icon.png
+├── icon@2x.png
+└── icon@3x.png
+```
+
+
+```javascript
+var appIcon = new Tray('/Users/somebody/images/icon.png');
+```
+
+Following suffixes as DPI denses are also supported:
+
+* `@1x`
+* `@1.25x`
+* `@1.33x`
+* `@1.4x`
+* `@1.5x`
+* `@1.8x`
+* `@2x`
+* `@2.5x`
+* `@3x`
+