If I don‘t subscribe to Image Hosting Plus, can I still include my own images in emails?

Yes!  You are not limited to using only Benchmark's image service to include images in your email.  As long as your image is hosted online and you are able to get a URL for it, you can include it into your email.  Benchmark does provide 10 MB of image storage space for all accounts, including free, and this will allow for the convenience of inserting your images in your emails.