Read http://developer.android.com/guide/components/intents-filters.html for an intro on how to do this. Search from "image/" on that page.
You need to declare the file types in your application's manifest file. Be sure and declare them using a mimeType like "image/*" or "image/png" so that you can open the files from other applications like gmail.
In your activity you would add something like this:
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
<data android:mimeType="image/*" />
</intent-filter>
The other catch is that you also have to handle both "file" schemes and "content" schemes. To see what I mean open a file from Gmail in Jellybean and you will see that the URI you are provided is a content type instead of the typical file type.