to post from desktop you have to "Upload attachment" not insert image.
There is a tab for this below the "Submit/Preview" button
Attached photo files are a little ungainly here. they appear in a widow and you may have to scroll left/right/up/down.
To make images appear cleanly, you can insert a Img url from a hosting site.
The site "Imgur.com" is a good one and free. It lets you easily choose the size of the picture