You're asking the wrong person. Due either to my client, or my instance, I get, like, a 20% success rate on attaching images. Most of the time I get an error.
On my client, there's a little picture icon. When I click that, it lets me select a picture and, if the wind is blowing in the right direction and the stars align, the picture is embedded. Otherwise, I upload to catbox.moe and manually embed the URL. The markdown syntax for an embedded image is: