Thanks for reporting this. It seems there is a bug with new installs where the Microthemer link in the top admin toolbar doesn’t appear. It should appear in the left menu of the WordPress dashboard. And if you go to the Preferences page and save the preferences, it should solve the issue:
Hopefully that will tie you over until I release a proper fix. Please let me know how you get on.
Though you can add background images to elements already on the page with Microthemer, it’s not a content creation tool. If adding images via the WordPress Post or Page editor isn’t right for your purposes, you might want to try a page builder. Page builders make it easy to create column layouts with drag and drop.