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:
I use Jerboa client on Android. It feels very much like the Reddit is Fun app.
There are rich text buttons at the bottom of the new post field. One is a photo icon, click that and it takes you to your photo/file browser to select a pic.