Depends a lot on what art style you're going for (vector, pixel, hand-drawn, etc.) and whether by "Sprite" you mean a still image or an animation with multiple frames. In addition to the previous suggestions:
I haven't used it myself but I hear people talking a lot about Aseprite for pixel art/animation, it's not free but they do have an Ubuntu version.
For vector animation, Synfig is an option which you can likely get through your distro's package manager.
