say i have a small patch of an image. in order for me to use that was a desktop wall paper on a pc screen i would normally turn on the “tile” option when setting it as a wallpaper. the default background image used by GDM (/usr/share/pixmaps/backgrounds/gnome/background-default.jpg) is a horrible green leaf and i hate it so much. despite some modest searching i could not locate a minimalistic dark textured background.
i found a small image of size 171×206 pixels that was to my liking and taste. next i found out that the horrible background-default.jpg image of of size 3072×2048 pixels. this is what i did to obtain my new GDM background image
composite -tile sample.jpg -size 3072x2048 xc:none background-default.jpg
then simply overwrite the old image with the newly created image. composite is part of the wonderful and amazing ImageMagick suite of tools.