This depends on their email inbox and the client they use for viewing their mail. This is normal for embedded images. base64 encoded images are actually blocked by most web mail clients, and that's what you've got there in your screenshot.
A safer way to embed an image would thus be hosting it somewhere online and then embedding it via a URL, instead of a base64 encoded string. This will usually not be blocked by most clients.
An example code for that would be as follows:
<img src="/wp-content/uploads/2018/11/blog/-illustration-email-embedding-images.png?w=640" alt="img" />
If your image is static, you can just host it somewhere manually and use the URL. If it's dynamic, this will require that you build the actions to host the image first into your flow before you can actually use it in the email body.
See this article for reference on the different ways to embed an image into an email: https://mailtrap.io/blog/embedding-images-in-html-email-have-the-rules-changed/
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