I'm waiting for this to be released: https://nvultra.com/ (I think it's limited to Mac/iOS, though)

I think https://simplenote.com fits all your requirements except images. I streamlined images by using services that allows pasting images into the browser (Trello, Glitch, Imgur), then get a link to the image with a single click. (If you abuse StackOverflow's question editor, they even give you the markdown for the image.)

Recently, I have been considering switching to Apple Notes after discovering you can access your notes from a browser. It fits all your requirements except markdown.