Which Graphic Formats Are Best?
For text and other graphics, particularly those with sharp edges or thin lines, we recommend using .png format (alternative: gif).
For images of people, landscapes, etc. we recommend using .jpeg files.
It is best to upload images in the size that you want them to be in the mailing. This way, they don't need to be scaled and will remain sharp.
Otherwise: Scaling up is worse than scaling down, so it's better to upload an image that is too large rather than too small.