I know Snapchat cracked down on 3rd-party apps when it comes to uploading pic/videos from the Camera Roll to Snapchat. However is Snapchat going to make an app like that because that was extremely helpful?
Unfortunately, there's no exact date or information related to the implementation of the Upload Pictures features. However, you have the possibility of using the online form to ask such features. Use the URL below and fill it with this request. If you're lucky, the developers will contact you to provide further information.
Features page: https://support.snapchat.com/co/other-feedback
Why use third party apps? you can snapchat upload from gallery derectly on your phone.