Some publications use submission forms, some use an email address, whereas others request drafts. You can generally find this info under the submission guidelines listed in a publication’s header or in its ‘About’ section. If not, you can always reach out to the publication directly via email. For more on this:

If You Are Looking to Write for Publications, Start Here

It’s an article written by Medium staff and covers some great tips for submitting to publications, including how to find ones that align with your writing goals.

I don’t know the answer to your problem with jpgs and curation... You could contact Medium and I’m sure they’ll quickly get back to you on that. Or have a look at Medium’s updated Curation Guidelines.

Hope it helps!