How to enable the user to change the background of the mobile application?

Question

css

Application built in PhoneGap from html+css+js.
The background is set in css:
background: url('../img/border.png') no-repeat;
How to allow the user to set the background from the file himself?
The simplest is if the user simply uploads the file to a specific, predefined location. To which place, in the case of Android and in the case of iOS?
Another - through the upload form, but again the question is where to load, how to rename, because the name is set in css.
If someone has come across this, please tell me how to implement it.

Answer 1

Via form + localstorage to save and jquery to change background in css