Path to my background images on IOS8 iphone or How to recover my background's images from my iphone
Assuming that you don't want to jailbreak your phone there are a number of utilities that let you browse through the backup of your iPhone and locate files, messages, voicemail and a whole bunch of other stuff.

Though I have not used it one of the top search results is iExplorer. And it is cross platform. 

There may be better ones but this says it works on Macs and PCs and you didn't say what you had.