The format shown in the help of gitter.im is , which solves the problem of including images from external websites, but not how to send images that are not already on a website. Does this mean we have to upload the image somewhere and then paste the link in this format, or is there a way to send the image directly through gitter.im?
Right, that is markdown syntax for creating a link to an image. You can use your own website, or a service imgur, or dropbox, or google photos, etc, to hold the images.