There're no specific requirements. To ensure the correct rendering of your images and graphic files in emails on different devices and in email clients, please note the following recommendations:
Make the image size twice the size of the email layout to ensure optimal display quality. For example, if the email is designed at 600 pixels, use a 1200-pixel image to facilitate rendering on high-resolution retina screens.
Don't use the SVG format that can cause problems with compatibility and performance.
Avoid adding large GIFs because they slow down loading and worsen the user experience.
Make sure the image retains its visual integrity in the dark or themed email templates for optimal visibility.
To learn the requirements of a specific target system, please refer to its documentation.