At default, all comic thumbnails use this graphic:
or this one if they're an A-rated comic:
First, you need to go to your comic's control panel (the Edit Comic area). Click on "EDIT" in the user sidebar:
You'll be taken to the Edit Comic page, where if you scroll down a little you'll see an option to upload your own thumbnail:
Click on the "choose file" and use the file browser which pops up to locate your thumbnail.
Guidance on thumbnail images:
Once you've chosen a file from your computer, the page will look like this... you MUST click the "SAVE" button to keep your changes!
You know you've successfully changed your thumbnail if, after saving, your chosen thumbnail shows up in the thumbnail image: