Make sure you add ALT tags to all of your images. I know it's a pain - I am also lazy, so I understand, but images don't always load. Even if you're GOOP.
Wondering wtf is an ALT tag? ALT tags are a small piece of HTML code that describe the content of an image and they're important for a couple of reasons.
First and foremost, they help people with disabilities access your email. If an image doesn't load for someone using a screen reader, the ALT tag will provide a description of the image so that they can still understand the content of the email.
And, if your images don't load, the reader can tell what was SUPPOSED to go there. You can see how descriptive the ALT tag is in the Goop example.
RE Images - make sure they are optimized (but still retina!)
But how do you make sure your images are loading quickly in the first place? You can use an app like webutils or a number of other image editors to optimize them before you add them to your email (or scripts > load images into stack iykyk). Make sure they're at least 1200px wide for RETINA.
And use a testing tool like Litmus or Email on Acid to QA your emails before you send them out. These tools will let you see how the email looks on different devices and in different email clients, so you can catch any issues before they reach your audience.
Two fairly simple things that will improve the user experience and accessibility of your emails. Make sure to use a testing tool to QA your emails before sending them out to catch any issues. Trust me, your anxiety will thank you!