Huh? You can paste if you have an image in your buffer, or drag-drop image files into an email in the Fastmail composer. I paste images into emails from screenshots almost every day.
(I'll take this report as a "we need to make it clearer you can do this!")
Very happy Fastmail user here! Would love if images could be resized in the webapp. For some reason most screenshots I paste in get scaled up to a very unwieldy size.
(I'll take this report as a "we need to make it clearer you can do this!")