If you didn't create a backup of those pictures on a cloud or some alternative storage method that can be accessed remotely, then the pictures are lost forever. However, if the pictures were stored on the external storage and you have that in form of a MicroSD, then insert the card into another phone and install the HidePhoto app again.
On the Play Store application, the software is listed as having support for backup and sync on cloud.
"Hide Photos supports local backup and cloud sync. So you can recover the photos and videos easily when you change phone or delete by accident."