Sencha have a service for this called Sencha.io.
Look at the image data conversion description here:
The basic use:
This will return the image in Base64 format.
Alternatively as a JSONP Callback (to App.myCallBack including the id reference "idRef"):
imageAsBase64 = "http://src.sencha.io/data/" + imageAbsoluteURL
You can also pre-convert your images to Base64 and have them listed in your CSS or code. Now your image is in a data format, it can then be stored with localStorage.
http://www.sencha.com/learn/taking-s...-apps-offline/ is a guide showing the mechanics of setting that up.