Hey Liz!
The icon is there - and I've definitely clicked it several times, to no avail. However, I've just discovered the issue. In the past, we'd click on the image, then click on the left or right alignment icons.... when you do that, the image will move to the left or the right. HOWEVER, if you have the image selected and click on the center icon, it won't center. Peculiar!
That said, I just created a quick test email and after clicking around further, the center icon works, but only if the cursor is to the left of the image (rather than having the image itself selected.).
So, problem solved (glad to not have to use the tables any more)! However, if there's a way to make the icon work when the image is selected (as it does for left/right icons) that'd be great!
Thanks for chiming in! - Jen