In Ghost, the markdown for image is

Is there a hidden format for adding a class in the img tag? Or is this feature not in G
In plain markdown classes for images are not possible. In some implementations it is possible, e.g. markdown extra uses  {.class}.
I tried it, but in Ghost it is not possible, neither . Also no hints in the doku.
But you can use a workaround if you use the src as css-attribute.
Just add an 'useless' hash to the url:

and adress this in css or js
img[src$='#img-thumbnail'] { float:left;}