Originally Posted by jefro

Not so simple.

"Note:

Since GNOME 3.16, GNOME Shell themes are now stored as binary files (gresource).
This change will be overwritten on subsequent updates of gnome-shell.

Firstly, you need to extract the existing GNOME Shell theme to a folder in your home directory. You can do this using the following script:

extractgst.sh

#!/bin/sh
gst=/usr/share/gnome-shell/gnome-shell-theme.gresource
workdir=${HOME}/shell-theme

for r in `gresource list $gst`; do
r=${r#\/org\/gnome\/shell/}
if [ ! -d $workdir/${r%/*} ]; then
mkdir -p $workdir/${r%/*}
fi
done

for r in `gresource list $gst`; do
gresource extract $gst $r >$workdir/${r#\/org\/gnome\/shell/}
done

Navigate to the created directory. You should find that the theme files have been extracted to it. Now copy your preferred background image to this directory.

Next, you need to create a file in the directory with the following content:

gnome-shell-theme.gresource.xml

<?xml version="1.0" encoding="UTF-8"?>
<gresources>
<gresource prefix="/org/gnome/shell/theme">
<file>calendar-today.svg</file>
<file>checkbox-focused.svg</file>
<file>checkbox-off-focused.svg</file>
<file>checkbox-off.svg</file>
<file>checkbox.svg</file>
<file>dash-placeholder.svg</file>
<file>gnome-shell.css</file>
<file>gnome-shell-high-contrast.css</file>
<file>icons/message-indicator-symbolic.svg</file>
<file>icons/pointer-double-click-symbolic.svg</file>
<file>icons/pointer-drag-symbolic.svg</file>
<file>icons/pointer-primary-click-symbolic.svg</file>
<file>icons/pointer-secondary-click-symbolic.svg</file>
<file>key-enter.svg</file>
<file>key-hide.svg</file>
<file>key-layout.svg</file>
<file>key-shift-latched-uppercase.svg</file>
<file>key-shift.svg</file>
<file>key-shift-uppercase.svg</file>
<file>no-events.svg</file>
<file>noise-texture.png</file>
<file>filename</file>
<file>no-notifications.svg</file>
<file>pad-osd.css</file>
<file>process-working.svg</file>
<file>toggle-off-dark.svg</file>
<file>toggle-off-hc.svg</file>
<file>toggle-off.svg</file>
<file>toggle-on-dark.svg</file>
<file>toggle-on-hc.svg</file>
<file>toggle-on.svg</file>
</gresource>
</gresources>

Replace filename with the filename of your background image.

Now, open the gnome-shell.css file in the directory and change the #lockDialogGroup definition as follows:

#lockDialogGroup {
background: #2e3436 url(filename);
background-size: [WIDTH]px [HEIGHT]px;
background-repeat: no-repeat;
}

Set background-size to the resolution that GDM uses, this might not necessarily be the resolution of the image. For a list of display resolutions see Display resolution. Again, set filename to be the name of the background image.

Finally, compile the theme using the following command:

$ glib-compile-resources gnome-shell-theme.gresource.xml

Then copy the resulting gnome-shell-theme.gresource file to the /usr/share/gnome-shell directory.

Then restart gdm.service (note that simply logging out is not enough) and you should find that it is using your preferred background image.

For more information, please see the following forum thread. "

https://wiki.archlinux.org/index.php/GDM

Originally Posted by mddnix

In Fedora, there is an easy way.