1) Using a browser (Chrome, if possible), go to this case-sensitive URL: http://WO-SIGNAGE-CMS/ContentManager
Login with these credentials for now:
http://WO-SIGNAGE-CMS/Content Manager is an internal web site that may only be accessible from specific locations.
2) Click on "Media"
3) You are now in the Media Library, which holds all your logo images. To delete an image, select the files you want to delete. Then click on the "Delete" button.