If there isn't a cover in the comic file yet, you can do it directly from the comic detail - if not, you'll have to edit the file to modify it.
Add a cover:
- Drag the image you want to use from your computer or another browser tab and drop it in the place of the cover and...voilà; the magic of the internet does the rest!
- If you prefer, you also have the option to do so by selecting an image that is on your computer by clicking Select the cover.
Change a cover:
If the comic already has a cover image, you must enter the file to be able to modify it.
Tap 
to open comic book options, tap Modify, and you're in. Now you can:
- Drag the image from your computer or another browser tab.
- Select the cover and upload it from your computer.
On some tabs you can't change the cover directly, but if the image is wrong or you have a higher quality one, you can add it by tapping