This is pretty comprehensive from the learning center:
https://litmus.com/community/learning/25-understanding-background-images-in-email

I'm not sure if you've looked into Mad Mimi. It's fairly cheap and pretty straightforward/ user-friendly and they have great customer support. Drip campaigns are pretty easy to set up. I'd look into at as an option. I refer to it as MM in the office because Mad Mimi sounds ridiculous, but that's it's main flaw.

I get a really high click-through rate on this link, too. I assume it's because someone wants to leave it open in their browser and save it for later.