Greetings and thank you for the support!
We are using the standard Steemit posting platform and it's built in Editor... really nothing special. The whole thing is some additional pre-work and images design in CorelDraw and Photoshop. After the designs are created - we are ready to "Submit a Story". It's simple as that!
Hope we were useful :)
P.P. At the moment we are using https://imgsafe.org/ for uploading our work.