By default, the option to Optimize large images for email is turned on in all accounts. It is a feature designed to prevent you from uploading images that are too large to be used in email Campaigns.
This is because when recipients receive Campaigns containing large images, they experience delays while the image loads. Even though the image may appear smaller when viewed in an inbox, our system does not actually resize the image itself (we only adjust the display size of the image). So each recipient's device has to download the originally sized image, and for large images, this delays recipients being able to view your images.
To avoid these issues, we automatically resize any large images wider than 800 pixels proportionally to 800 pixels wide on your behalf when they are being uploaded to the Content Library.
We do not however resize any images with an extreme width:height ratio greater than 8:1, as we understand such images may have been created for a specific purpose.
If your Campaign requires an image that exceeds 800 pixels wide, you can deselect the option to Optimize large images for email and no modifications will be made to images uploaded to your Content Library. However we would also recommend contacting our support team to discuss your needs further so that we might be able to provide additional guidance in terms of optimizing your Campaign content for the email channel.