This "grey checkered background" is called the root weave, it is configured by the X server.. not the window manager.
Upstream Xorg developers have mostly decided to transition to a "black root window" instead of the classic weave, but OpenBSD's xenocara retains this.
For gradient images, they are plenty of ports capable of setting a image.. xsetroot can be used from your .xinitrc/.xsession files to configure a solid colour, read the man page.
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