When user signed up, it will show the default logo before uploading own logo by the user. To set your own logo update the Workspace model ( Details of the Workspace.php file can be found in the configuration guide )
The logo files are located in -
Option 1: Replace the logos from the above folder.
Option 2: Or put new logos here and update the names in Workspace.php file, for example-
[ 'key' => 'logo_default', 'value' => 'logo_2105025689.png' ], [ 'key' => 'logo_inverse', 'value' => 'logo_inverse_7627893866.png' ],