How to Set a Background on a Kindle Fire
The Kindle Fire HDX runs on an Amazon-centric version of the Android operating system. As such, this device doesn't offer as much customization capabilities as other comparable tablets, which means that you can't change or choose your Kindle's background or lock screen images.
1 What You See Is What You Get
A lock screen appears when your tablet goes to sleep to save power. This screen shows ads for users who opted for the version with "Special Offers," and cycles through a series of stock images for those who paid to have the ads removed. These images are provided by Amazon, and you cannot change or alter them in any way.
2 Root for Change
Because Amazon designed this tablet as a content-driven device for use with their website, there's no room for personal customization. To use your own photos for lock screen backgrounds, you need to root the device first to gain access to the tablet's operating system code. This could at best void your warranty and at worst render your tablet unusable, and should only be done by experienced users.