Well, in traditional pen and paper animation. tweening is just adding another frame between two frames and drawing it, and doing it over again until when each image is display back to back in short succession it creates the illusion of fluid motion.
one way of artificially tweening is to alpha blend between each frame a extra one. for example, frame 1 and frame 2 have an additional frame placed inbetween which is frame 1 at at 50% opacity/transperency sitting on top of frame 2. your can add more transitions as you like to make it even longer just like regular animation. typically the more frames you add the shorter you want the duration each frame gets displayed.
Rinse repeat for each frames in order.
This will sort of "blend" the frames into one another.
you can hard code this into the animation by manually doing this in your image editor. or you can do it within your game engine. there is alway the option to code it yourself as well.
I think of this like interpolation of the frames.
Now personally. I like to adjust how long each frame is shown(usually in milli-seconds), make it longer or shorter till the animation feels right easier.
I usually make my stuff in PhotoShop. and then code the animation graphics in C++ and SDL2 and SDL_image.