In plain markdown classes for images are not possible. In some implementations it is possible, e.g. markdown extra uses ![alt](src) {.class}
.
I tried it, but in Ghost it is not possible, neither ![alt|img-thumbnail](src)
. 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:
![alt](src#img-thumbnail)
Then you can address this in CSS or JavaScript:
img[src$='#img-thumbnail'] { float:left;}