Mailmeteor is built on the native Gmail editors. It means that you can copy paste images in your email template and add emojis very easily.
To insert images, you need to upload them to a server before. You can either upload images to your own web server or use free services likes ImgBB or Imgur:
Once uploaded, go to the image's link, right click and copy image, then paste it to Mailmeteor template.