Tutorials on Creating SplashScreens:

(Note:The First Part is actually 3 parts, all about extracting.)
Part 1:Extracting Downtown Splash
Requirements:A Program that Can extract Files from .FAR Files. Some examples would be FARx, or Far Out.
1)Open your Far Extraction Program (I'm using FarX, since I know what Files I need to extract) and open UIGraphics.far in /UIGraphics in your Sims Folder.
2)To get the Downtown Splash Screen Extracted double click on the Downtown Folder in FarX, or scroll down to the Directory Downtown/ in any other Far Extraction Program
3)There should be many files, but we only need one of the 2 of these Files
-Taxi_loadscreen_800x600.bmp
-Taxi_loadscreen_1024x768.bmp
Choose the one for your Resolution you Run Sims At (If you Run it normally from a shortcut, You're probably running it at 800x600.)
4)Extract it to THIS directory. UIGraphics/Downtown.

Part 1.3:Extracting Vacation Island Splash
Requirements:Same as above
1)Open your Far Extraction Program and open UIGraphics.far in the /UIGraphics in your Sims Folder.
2)To get to It go to the VIsland Folder, or Directory as said above
3)OK, there are 2 different Loading Screens which are in 4 files. These are....
-vacation_loadscreen2_1024x768.bmp
-vacation_loadscreen2_800x600.bmp
-vacation_loadscreen_1024x764.bmp
-vacation_loadscreen_800x600.bmp
Choose the two ones for the resolution you run sims in, Extract a loadscreen, and a loadscreen2. The Loadscreen2 is the Ship one. The Loadscreen is a Shuttle.
4)Extract it to THIS directory. UIGraphics/VIsland

Part 1.6:Extracting Old Town Splash
Requirements:Same as the Other 2
1)Same as Step one for the other Parts before this.
2)To get it go to the Nbhd Directory or Folder
3)Ok 2 Files AGAIN
-Bus_loadscreen_1024x768
-Bus_loadscreen_800x600
Extract the one for your Resolution
4)Extract it to THIS directory. UIGrapghics/Nbhd

(This part is really confusing.)
Part 2:Replacing
Requirements:A Paint Program of Some type, even Microsoft Paint will work, but you won't get the best results from it. Or a Screenshot, Or Picture. Or Something.
1)Ok, now you have those original Files Extracted. Now it's time to mess with them and change them.
2)You can go ahead and clear the Image if you want. Or you can just write something on it. The main point of the Whole Extraction Part was to make sure you extracted the files and knew what their names were
3)You can also use a screenshot. Make sure It's the Same Resolution of what you're replacing though.
4)Or a Picture.
5)Or anything you want.
6)Do the Same for all of the Files you want to replace

Part 3:Testing
1)Open The Sims.
2)Go Downtown, Old Town, Vacation Island
3)See if it's there. If it is Congrats, you made your own Splash for DT,VI, or OT! If Not, Try again. 
VI=Vacation Island OT=Old Town DT=Downtown.