A few tips and tricks that come to mind:

-I forget the exact number of pixels, but iPhone tends to clip the image a bit, so make sure you do test sends on a few devices to doublecheck

-Do some experiments to understand scenarios where the image helps you, and anywhere it either does nothing or even lowers your desired conversion rate.

-MMS do take longer to send than SMS. I would recommend giving yourself a GENEROUS buffer if sending later in the day (to avoid any inadvertently getting received after quiet hours), and SMS is the better option if you're sending on a high-volume day like Black Friday or Amazon Prime Day. (A way to navigate around this--I used to pre-seed/tease with an image and a short message the day before so the image would still be visible on the screen when we sent a follow-up SMS on the high-traffic day.)