Do you know Gimp? Open source image editing tools such as Photoshop. It has the ability to create transparent photos like Photoshop, you can simply take your own photo, process the background transparent image, import the processed image from Powtoons, and synthesize it into the animation. Gimp can be downloaded from http://gimp.org.
Powtoons also provides free access to basic services. The following video shows you can easily create animations with Powtoon.
You can use the functions of Powtoons and Gimp to make the following video.
I also discovered the Openshot video editor, an open source video editing tool, which has the function of image insertion. Combining this function with the previous Gimp function seems to have a lot of interesting effects. Openshot can be downloaded from http://openshot.org. The following tutorial will help you understand the functionality of Openshot.
I use Screenflow on my Mac, and the video chroma key feature is great. OpenShot also has a chroma key function.