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.
Salesforce Dashboards can provide easy to understand, quantifiable data on Marketing Cloud email sends and Cvent events - among other things. When used in combination with other information and your own criteria of success, it will help paint a more complete picture of your outreach efforts.