I've had a workflow installed for a while (I think it was created by Tim Nahumck) that uploads images from your library to WordPress after letting you rename them. It then gathers the URLs to the uploaded files and handily puts them into a Drafts item for you.
I added an extra step to also convert the images to .png as part of the process.
Looks like I might be giving it a spin as I think it will be the quickest way to work with images and apply the Markdown Extra classes.
You can grab the original workflow here.
(Note: you need to change the path to your uploads folder manually in one of the steps.)