In some of the lessons, there is a balloon sprite. In other lessons there isn’t. In Lesson 17, the example uses a balloon but there is no balloon when students click on new animation. There is a balloon sprite in Lesson 14, #11. Is there a way to copy and paste it into lesson 17?
To use an image that isn’t in the library, you would need to upload it yourself. One place where I’ve been pretty successful finding images that work well is at http://safesearch.pixabay.com For example, here’s a red balloon that would work:
You could also download an existing image from the animations tab (export as a .zip and then open it to get the individual .png file which you would then have to upload into a new project) or you could remix a project to keep its images and then delete the code and use whatever code you want to use.
I realize none of these solutions may be ideal, but for me finding an image on a site such as pixabay works well.
Hope this helps a little.