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#msg1557543
If 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.
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.