I've found a site to create comics with these heroes (if you click on the image you'll see it).
I really liked it, it's easy to use and you can easily save the comic or print it.
The problem comes if you want to post the image in a blog to show your student's creations. The document you have when you save the comic is a PDF and I had some problems to post this image.
Image from Marvel
Nevertheless, I think this page will encourage my students to write some short stories in English.
Great idea from Marvel.