You might have encountered a malfunction of the application. To fix this, go to Settings > Application > Manage Application > All tab, select Voulty and choose Clear Cache. Open the application and see if the picture is still there. If it is, in order to completely delete a photo, try unhiding it first. After you did this, go to Gallery on your device, find the picture and delete it from there. It's also advised to keep your application up-to-date at all time to overcome this kind of behavior.