In Home > System > Settings> Notifications > Common Notification Settings the URL to the logo is set in a format like:
It used to work correctly but after upgrade to OSA 7.0 the url shows
404 Not found error, as a result notifications are sent without logo.
The link with logo.gif has been configured a while ago, when skin version 2 was used. Since OSA 7.0 support of skin v2 was dropped in webgate, as a result the corresponding rewrite rules were removed from branding_htaacess files making such URLs inaccessible.
Change URL to logo to: