Whenever someone recommends a command you are not familiar with read the man page first so you can tell what it will do before you run it. In time you will built up a useful set of commands.

Why not just copy the image you want over to /usr/share/slim/themes/<theme_name>/? Then just delete the current image (or rename it to something else) and change the name of the new image to background.png/jpg.

Simplest way right there. But it absolutely helps a LOT if you have a couple of right-click context items, Open as Root and Edit as Root.

I haven't used slim in many years, can you not just specify a specific image in slim.conf?

A few words of encouragement: the more you start to play with "Linux," the more you'll start to understand things. Everybody was a beginner once.

Did you also know you can put multiple folders in /usr/share/slim/themes/ and make an edit to /etc/slim.conf so that you can have multiple images it randomly chooses from? That's what I do.

I tried but it didn't work. I had to make a mistake because when I restarted, the screen was black. And I didn't know what to do (I'm a beginner), I had to reinstall Excalibur and it takes me some time. So I'm gonna wait a little while before I make any other changes. Curve-Darkpurpy on login screen works for me right now (It's dark so it's cool for my eyes).

My request is somehow partially resolved. I'll try to come it back later. Thanks to everyone for your help

Then if I chose a personal image instead of a preinstalled theme as background, what command should I do ?

Probably the easiest way to achieve this is to set your current_theme to "original" (in /etc/slim.conf).  Then in a terminal, run

cd /usr/share/slim/themes/original
sudo mv background.jpg background.backup
sudo cp $HOME/newbackground.jpg ./background.jpg

That is, assuming the background image you want to use is called "newbackground.jpg" in your home directory.  Replace that part of the last command with the actual file name and path.

Then if I chose a personal image instead of a preinstalled theme as background, what command should I do ?

I know there's a way to do it with LightDM but I don't want to install it.

For example, typing in sudo nano /etc/slim.conf will escalate user privileges by allowing you to directly modify the respective file. Make your changes, and then save with Ctrl+O. If you want to try a different preinstalled theme, you can then use cd /usr/share/slim/themes/ and type ls within that directory to view the available themes to choose from for the configuration file.

And when I try to edit /etc/slim.conf, I can't save because the file is in "read-only mode".