Starting from version 6 iOS does not allow apps to save photos if photo access is disabled in the Settings. I assume it is the source of the problem.
Please go to the Settings App -> Privacy -> Photos and check whether photo access is enabled for LiveSketch.
And make sure you are using the last version of the app.
If it does not help, please provide me more information about your device model so I could reproduce the problem.
Please send me your skethes at livesketch[at]rabovik.ru. The best screenshots will be used on the app store.