EDIT: silly me. I forgot to edit stars.tbl
Oh dear.

Please, DO NOT edit retail files. I suppose it looks easier, and it's natural for a modding newbie, but...just don't.
What you do, is create a new folder in your /freespace directory, and name it /mymod, or /Mort_s_Mod or /reallyawesomemod or whatever. Then, you create a /data folder inside it and /effects and /tables folders inside
that. So, the end result looks like this:
/freespace
/data
/effects
/tables
Then, you place the bitmap you want to add to the backgrounds in /effects. You then write a .str-tbm with its name and add it to the /tables folder.
Then, to run FS with your mod enabled, you go to the launcher and select your mod.
See
here for modular tables (tbms)