Login Screen Background is not changing.
As far as I know, there is no straight forward way to achieve this. The tool that can be used for this is LightDM GTK Greeter Settings. But this is not available for Solus.
Another way is manually change the image from background folder. (got from solus forum)
1. Rename your .jpg file to TullamoreGrandCanal.jpg and place it in pictures.
2. Open terminal and run:
sudo mv /home/your_username/Pictures/TullamoreGrandCanal.jpg /usr/share/backgrounds/solus/
3. Restart and enjoy.
Graphical utility LightDM Greeter Settings is now available in solus repository. Now its easy to change Login screen background. See the below post