Paint Online with Sketch and Paint. Pretty sophisticated program with many choices of shading, pen/brush size, pressure, etc. Color choices can be basic or with an unlimited pallet. Program uses Flash, so some devices may not work. Children can use it easily, but it is sophisticated enough for adult use.
Choose the image size at the beginning. Files when complete can be saved as a .jpg and downloaded to your desktop. You can also open an image already on your computer and add to it.
Best of all there is no login required, no need for students to register and have an email address.