I am searching for a possibility to get better cover art from Audible.
The default image is about 300px and the url is something like this:
http://ecx.images-amazon.com/images/I/<someRandomId>._SL300_.jpg
If you change the "300" to any number between 1-500 you get the same cover art in the specified resolution. Changing a number higher than 500 results in getting the 500px version. While 500px is certainly not enough, it is definiteley better than 300px.
I also found out that there are several reverse image search engines that you can use to find a better version of an existing cover art you already have. You can use Google, TinEye and Yandex. In my experience Yandex gives the most results. It also displays the varying resolutions on top of the site, so you are able to select the version you want relatively fast.