Q:

How to make SLiM not render anything at login prompt?

I've just figured I don't actually need an image when logging into slim, especially if that image is a few megabytes large.
Currently they're both set to 1x1 png images, however, then that image has to be rendered a few million times each frame, which isn't quite efficient either.
How can I disable both the background image and the panel image in SLiM?

A:

There is no way to turn if off using config files. It is hardcoded in sources (with names of files), so only way to remove background will be to modify sources.

My background is about 150KB, panel about 15 KB, so it lot smaller that "few megabytes". 
You can set in your slim.theme:
background_style stretch

so if its 1x1 px there will be only one stretched file and there will be only one copy of this file.
BTW, turning off shouldn't make any visible difference in used resources of modern computer. If you want to run X applications there will be apps that are using many more megabytes of graphics...

