ID3 covers from Rhythmbox not being displayed

Thanks for your answer. So basically what CoverGloobus does, is look in the same folder as the music file for an image file, and use that as the cover image?

yes
it checks the folder for cover (folder.* cover.* album.* albumart.* front.* %artist% - %album%.* )
if nothing found:
it checks if player sended and uri of the cover
if nothing found:
(if download from Amazon option enabled) it tries do download cover from the web and saves it in cache for later use

Ah, I understand now. Thank you very much for the quick answer!

Thanks Alexandr Grigorcea, that solved my question.

I switched to Banshee and noticed the same result, that is, covers from ID3 tags are not being displayed. But I recently started using Docky as my primary dock. Docky includes a Helper plugin for Banshee (/usr/share/docky/helpers/banshee_control.py) that displays an icon of the cover on the dock. But in contrast to the CoverGloobus plugin (/usr/share/covergloobus/players/Banshee.py), it shows every cover, including the ones that come from the ID3 tags.

With my very limited Python knowledge I compared the scripts. It seems that both are using the same method:
They receive the player and cover info from the dbus (whatever that is). This dbus thing apparently returns an "artwork_id". This id is then converted to a path to the cover image: "~/.cache/album-art/{artwork-id}.jpg"

Since both are using the same method, CoverGloobus should be able to do it as well, without having to look in the ID3 tags. So Banshee.py must be missing something that's causing this. I tried the figure this out myself, but the Python scripts are a little bit too complicated for me.

Maybe someone else can have a look at this.

I did it! My first bugfix =P
I made the bug report with the patch:
https://bugs.launchpad.net/covergloobus/+bug/549503