From "The Email Design Best Practices You Need to Know"

Photos give you the opportunity to brand your emails, make them more visually interesting, and get constituents interested in your offerings. Regardless of what you want your photos to accomplish, you need to make sure they display properly in emails.

Photos and graphics should have excellent resolution no matter their size and high quality enough to get a point across — but not so large that they slow down your email’s load time. You should also consider whether or not they actually add something essential to your emails. If you’re just using them to take up space, you may be slowing down the loading process and wasting valuable real estate. You can see more specifications about header and email image recommendations here.