In order to do that, you have to make what's called a "Skybox", a six sided box which is created around the space the player flys around in. Each of the 6 faces is a separate texture which will consist of some cut up version of that eagle nebula picture. It takes a fair bit of work for someone new to make a skybox from scratch, but if you're motivated you can do it.
Lucky for you, someone already made an eagle neubla skybox:
http://www.hard-light.net/forums/index.php?topic=67899.msg1557543#msg1557543If you open the file and open the folder that reads 'data' you will see 4 folders: cache, maps, missions and models. Go into your freespace2 root directory and place the maps folder in freespace2/data/maps Place the mission file in freespace2/data/missions and Place the .pof file in freespace2/data/models. You have 2 options here.
Option1(may not work with the TBP mod):
Start the game and open the mission titled "Eagle Nebula Skybox Text" and fly around in there. You should see the skybox in the mission.
Option2:
Open the mission editing program titled fred2 and select a mission. Open an existing mission or create a new mission. Open the editors tab and select background. You should see a text field in the bottom right where you can type the name of a skybox model. Type: eaglenebula.pof Then save the mission.
Once you have done that, open the mission you created or modified and you should see the eagle nebula skybox.