Here’s some brief instructions on how to add a Protractor to your Explain Everything screencast videos. I’m sure this probably works in other screencasting applications: the key is to use one that allows you to add multiple layers/images. If your layers get overlapped incorrectly in Explain Everything, you can hit the i button and rearrange the layers.
This will work with any transparent image. If you can’t find a transparent image you can use Photoshop to create one. (I’m not sure how long this tool will be free, but it’s another option if you don’t have access to Photoshop: http://clippingmagic.com/)