For this reason, try to be consistent in sending newsletters : better student data one email a day, than one every two months. You have to get users used to reading your emails, only in this way will you keep the opening rate high and - above all - avoid being labeled as SPAM.
Image Management
Ok, an email must certainly be captivating, aesthetically beautiful, with images at a good resolution and effective. However, pay attention to some specifics that can help
try to keep the image size around 50KB
check the size of the images, they must not be too big (they will weigh too much), nor excessively small (they will appear grainy).