My suggestion would be to oberlap two images. My example would be to combine a picture that was made with the camera with a pen input.
See here: https://powerusers.microsoft.com/t5/PowerApps-Forum/Combine-pen-input-with-a-picture/m-p/48320/highl...
This would be very useful and I have been needing to achieve the same thing.
I have had to store the images in two separate blobs in a db, to retrieve them later into two image objects one on top of the other.
The other alternative is to write a c# web service which will take the images and merge them together.