If you're using libGDX you could use a spritesheet, it means all the images of the animation in the same png (you could also use separate images if you want).
Then the animation should be done with the lgAnimation class.
This way is how it's done in one of the libGDX examples, and that's the way I've allways done it.
I agree we could have more examples and code, but if you're using libGDX (almost mandatory in game making with b4x) there are plenty of examples provided, that's the way I learned. I know the learning curve is hard. You have to study the examples and play with them until you know how things work...