If you have associated images (like fan art, pinups, navigation buttons, etc) which you would like to upload and use for your comic, you can use your Image Locker to do so.
The Image Locker is associated with your user account, NOT with any specific comic.
To access the Image Locker, first click on your userpic/avatar in the user sidebar:
You'll then be on your User Control Panel. One of the menu buttons says "images" - this is the one you want:
If you click on that, you'll be taken to your Image Locker. It will list all the images you currently have uploaded, and to upload another image just use the "Choose File" to select it, and the "Upload Image" to send it. You do have to do this one at a time, currently there's no batch upload process. Here you can see I am about to upload an image named "asback.jpg"
And here it is uploaded. You'll need that URL if you're going to use the image anyplace! Note that if I roll over an image's URL, it gives you a preview of the image there. Useful!
Images uploaded to the Image Locker should not be automatically resized or converted in any way.