The welcome page can show custom images which is explained in detail here. AWP also ships with a static image in case no custom images have been configured. To change this static image use your customizations folder as explained in the CSS editing chapter.
The expected path to the logo image is: \awp\customizations\mycompany\app\style\awp\assets\login.jpg
Restart the Cumulus Web Solutions or reload the AWP settings to ensure the logo is loaded correctly.