I do not know which set up you have. Are you using Visual Studio c++ ? or unity?
In case of the Visual Studio setup you would be using c++. You can google how to create a .jpg using C++ and the Astra SDK gives you access the the frame its processing during any given moment, this can be seen in the examples provided in the Documentation.
I do not have any insight on how to do it in Unity or Android but it should not be any different from how C++ handles each frame, which then can be grabbed and saved onto a file using the Unity/c# enviroment or Android/Java environment.