There are a few reasons that Benchmark, or any Email Marketing Service provider, limit that size of images that can be uploaded into an email. One reason is that smaller images tend to load faster in emails, so that your readers can quickly read the emails instead of waiting for an image to load. Because loading time is also dependant on the reader’s connection, we wouldn’t want to impede that any further with a big image.
Another good reason is the ratio of image and text. If the email appears to be image heavy with minimal text, this can increase your chances of being filtered into the spam folder. The optimum ratio of the image to text would be 60% images, to 40% text.
Last but not least, image size also contributes to the email. The bigger the email, the more suspicious it becomes for security programs. There will be several filters, firewall, and security set in place to detect viruses, malware, and spyware. If the email was big enough, it may have the mail be rejected entirely.