Webkit-based renderers support it, and AFAIK that means just iBooks, Kobo when using its kepub flavor of epub (but not in regular epubs, which trigger its inferior renderer), and software like Foliate. At Standard Ebooks we use MathML in our core source (see for example the Tractatus[1]) and during build it's rendered to PNG for maximum compatibility since so few ereaders support MathML right now.
MathML would be so much easier than images!