If you press space there are some mousetrail ideas. Click "remix" to make your own mouse trail. There are two sprites, one says "your mousetrail". You can design and make your own mousetrail there. There are 20 costumes, add more if you need more mousetrails. Draw a picture or select a sprite, either is fine. Then there is some code in the code area. You can leave the mousetrail the way it is or you could add special effects. There are instructions on what to do. Once you are done creating your mousetrail, backpack it and you can use it in your projects.
You can also hide or delete the "mousetrail" sprite and just keep your own sprite. You can change the backdrop by using right and left arrow keys if you prefer a light colored background