This page is about the icons used in gmusicbrowser sorted in categories:
The icons in use can be in png or svg format. Since the sizes of the icons (e.g. on buttons) can be customized in the layouts there is no standard size for icons. Therefore it is recommended to use/create icons that scale well.
The folders where icon themes can be found are /usr/share/gmusicbrowser/pix/ (for the ones that come with gmusicbrowser) and /home/$USER/.config/gmusicbrowser/icons/ (for user-installed icon themes).
The standard icon-set is usually found in /usr/share/gmusicbrowser/pix. Custom themes can be put in subfolders (subfolder equals themename) and can then be selected in settings > layouts > icon theme (or in gmbrc > IconTheme: $themename).
With gmb-vol and gmb-queue you may add as many $n numbers as you like, e.g. up to gmb-queue100 or gmb-vol100. Your working hours spent on these icons are the only limit.
Even though there are no standard sizes indicators scale well at 16x16px, for buttons at least 24x24px is recommended.
Since gmusicbrowser 1.1.5 there's some extra eyecandy for the Cover-widget: overlays for album covers or pictures when no album-cover is found. See this commit.
Example:
Cover(overlay=6×6:350×350:/usr/share/gmusicbrowser/pix/elementary/overlay.png,default=/usr/share/gmusicbrowser/pix/elementary/no_cover.png)
Users can also define their own custom icons and use them in layouts, e.g. for togglebuttons:
TogButton1(relief=none,size=button,icon=tree,widget=SongTree,togglegroup=1)
The icons displayed here are the standard stock icons from the gnome-project.
(No descriptions here, the icons are pretty self-explanatory. Otherwise look them up in the link to the gnome-project above.)