Hey Joseph,
It will depend entirely on Zapier and the email tool you’re using, you’ll need to dig into those docs. My guess is you are creating a plaintext email rather than an HTML one. You’ll actually need to write HTML and an <img> element to display your image.
In general I prefer to setup a system like Mailjet with a transactional email template, and then I can simply pass in the data needed from Zapier / Make automations. That gives much better design options even on the free plan.
Drop me a message if you need something built quickly.