You could use the base64 technique that Pode has already build plugins for.
In my app i use the "drag & drop zone" plugin to get images into the frame of a sprite via the load from URI event.
Then at the same time i save that base64 image string into the webstorage under a custom key.
I upload the contents of the webstorage via the dropbox plugin to dropbox.
Then i can get that same "savegame" with the images on any other computer.
I simply download the savegame with the dropbox plugin again.
And on download i set the base64 images back to the sprites.
If u want images from dropbox, then base64 is the way to go.
Combining the dropbox plugin septeven build with the plugins Pode build is key.
And very simple to set up.
Hope this helps