We did something similar a while back to send an invitation to a retirement party.

I believe our image was in .jpg. Nothing within the image was customized, just the email and the Dear ___ greeting stuff.

We ran a few tests with only in-house Email addresses, and that test group checked it on our computers and our phones to make sure it looked decent. Strongly suggest you do the same, as Andrew suggested above.

I think we also opted to make the image hyperlinked to open to a page in a browser just in case Email didn't display it well. Maybe that was a different merge Email.

I think finally in the end we sent in batches, sending to a filtered list of recipients in the first go, who were in my boss's inner circle, so that if there were any hiccups, she'd hear it first from them.