The Site URL field in the Administration area of Blackbaud NetCommunity is no longer used to generate the links and images in NetCommunity emails.  The URL with which you logged in before editing the email is now used as of version 6.15 an above.  When you use the link picker in Blackbaud NetCommunity, the system builds the link by appending a relative image reference to the URL you logged in with or on version 6.10 and earlier it remains the URL entered in the Site URL field.

If you are using Blackbaud NetCommunity 6.15 or above: 
Ensure you are logging in using the correct URL

If you are using Blackbaud NetCommunity 6.10:
The Site URL field can be found by browsing to Administration -> System Options.