Here's how I post them... I opened a imgur account (totally free, no limits) [optional: I set my photos to "Private"
].
At Graphic Audio Site, Right-click image, "Save Image As", save to computer.
At imgur, click "Upload Images" (I use Browse Option) select cover picture, click "Upload". On the right side is a list of sharing options, Click on "Direct Link" (copies Image Location to your clipboard).
At the Audiobookbay, paste link in "Cover URL" portion of the upload form.
For all actions above I opened new browser tabs so all sites are accessible at once.