I have resolved my problem using iExplorer (mentioned by @billyork23). In my case, it is the thumbnail data of old photos. You can locate it under:

Media/PhotoData/Thunbnails

and delete all .ithumb files. It saves me 1.5GB.

P.S. The iExplorer does not require jailbreak or anything.

I have resolved my problem using iExplorer (mentioned by @billyork23). In my case, it is the thumbnail data of old photos. You can locate it under:

Media/PhotoData/Thumbnails

and delete all .ithumb files. It saves me 1.5GB.

P.S. The iExplorer does not require jailbreak or anything.