I prefer animations over still images. Images are good for scenes, that don't really need animation. But for the lewd stuff, animations are a must for me
Good long animations are top tier. Still images can be great. Especially if they can convey a sense of motion/life. But a short looping animation, it's just such a bummer. I don't want paper dolls just going in-out, forward-backward. Add some hand motion, head turning, facial changes. Part of your animation can be moving between two points on a loop (in-out) , but have some other part be a 2-4 position loop. I'm not an animator, but there's gotta be some relatively low-effort options.
I'm fine with both. I don't like when games have 3 images that they cycle to "animate" things. At that point just add more images to the scene as it progresses.