For the newsletter Header, your image should have the same dimensions as your email header image, with a recommended width of 600px, and a maximum height of 400px.
For text modules, to ensure that a photo fills in the box in the body of a newsletter, consider that a newsletter/broadcast resizes an image to 640 x 320 pixels and preserves the current aspect ratio of the image. Articles with images equal to or larger than that image size with a 2:1 ratio are ideal for a newsletter.
Image With Text module has a limit of 300 character.
note
If you want to include a gif in a newsletter, we recommend using a gif with 170 frames or less for the best experience. The larger the file size of the gif, the lower the number of supported frames.