This app was mentioned in 4 comments, with an average of 3.25 upvotes
Actually I found a better app. The one you linked only allows you to use the front-facing camera (not the best) and also doesn't allow you to import photos.
The app is literally just called "Selfie" https://play.google.com/store/apps/details?id=com.argue.selfie
The default setting just puts a sketched face frame as the 'tracer' overlay but if you go into the settings you can set the last photo (either taken or imported from your gallery) as the tracer image and then change the opacity.
It seems to have a function that creates a video animation (either mp4 or GIF) but for some reason the videos created all seem to corrupt and I haven't managed to get it working. Nevertheless the overlay system works perfect and being able to import existing body selfies is great so that I don't need to start all over again.
Oh what a great idea to use a selfie app! I was looking around the suggested apps and this also came up. It apparently has the video option and it also lets me use the rear camera. https://play.google.com/store/apps/details?id=com.argue.selfie
Thanks so much!
https://play.google.com/store/apps/details?id=com.argue.selfie&hl=en
Not the same dev but i think that works!