Images used in Marketing Cloud emails need to be formatted to a certain size before being uploaded to Marketing Cloud. If you don't, the email might appear fine in Marketing Cloud when what's delivered to an inbox is completely different. Recipients of poorly formatted emails are less likely to read or engage and more likely to unsubscribe.
Wondering how much time elapses between your email send and constiutuents' actions? Use the Time Between Send and Engagement report in Marketing Cloud to help assess the need for reminders or additional touch points.
Curious if your readers are engaging with your content more on mobile or desktop? The answer to this question is key in helping you determine your most compelling email designs, and the Device Performance by Email Sends and Email Performance by Device reports in Marketing Cloud can help.