Hi, I’m wondering where I can drag my pictures in order to have them show up on the Backgrounds tab in the Appearances app. I thought if I just put them in a subfolder in /usr/share/backgrounds, they would show up…but they don’t. I know I can add custom locations manually, but I am wondering why dragging photos or subfolders into /usr/share/backgrounds doesn’t work.
Unlike the XFCE desktop, that alows you to select the background directory, Mate desktop stores background information (name and author) and settings (style, colors) in xml files. When you hover your mouse over the background image in the Appearance Preferences it opens a tooltip dialog that displays those informations.
<wallpaper deleted="false">
<name>Gulp</name>
<filename>/usr/share/backgrounds/mate/abstract/Gulp.png</filename>
<options>zoom</options>
<shade_type>horizontal-gradient</shade_type>
<pcolor>rgb(98,127,90)</pcolor>
<scolor>rgb(39,88,126)</scolor>
<artist>Daniel Galleguillos, Hylke Bons</artist>
</wallpaper>
When you add an image in the settings, those settings are written to $HOME/.config/mate/backgrounds.xml when you close the Appearance Preferences.
Like @elcste said there are xml files in /usr/share/mate-background-properties dir. Maybe adding a xml file in that directory will add them in the Settings.
What I find interesting in $HOME/.config/mate/backgrounds.xml is that one path points to the xml file /usr/share/backgrounds/cosmos/background-1.xml. In that file there are different settings like timing and transition that define slide show. There is on slide show, Cosmos in the Appearance Preferences. This is a good example on how to make a slideshow for mate desktop.
I moused over my background images in Ubuntu Mate 17.10 and /usr/share/backgrounds/Mate/name of picture, or /usr/share/backgrounds/ubuntu-mate-artful or xenial. Name os distro I guess, and will have artists name.