Best way to store large data clientside in an Vaadin-Touchkit application?

We have written an Vaadin7/Vaadin-Touchkit application where the user can select multiple images and upload them. The application works offline, so we want to store the images clientside and only upload them when the user is pressing the sync button. The clue is that we want to keep the selected images, even when the user is closing the Tab/Browser. The clientside storage should also work with mobile browsers (Android and iOS)

We have found the following possibilities to store data on the client

Session Storage is killed when the tab/browser is closed. Not suited for our case.

Local Storage is limited to 5MB which is not enough. Is there a way to extend that limit?

IndexedDB does not work on IOS devices (

WebSQL does work on IOS and Android devices ( but is deprecated

So the question are. Does anyone know a good Frameworks in conjunction with gwt? Which storage type/DB would you prefer and why? Or even more general, how would you solve the Problem?

Thanks for any advice, suggestion or experiences you had.