What is the best image format?

Use the JPG format.
Use the image in 100% dimension (do not scale height and width).
Make sure you add height and width attributes (use the built-in designer).

PNG and GIFs also work fine.
You need a GIF to use animated images.

The SVG format can be potentially harmful.

Attackers can easily exploit SVGs to embed malicious JavaScript code. That's why Outlook on the web might apply security policies that block SVG content. The fact that emails with SVG content are often classified as spam by email servers also aligns with the assumption that the format might be generally unsafe.

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