I'll do a breakdown for you to make it simple:
SetupLoadScreen(string&asTextCat, string&asTextEntry, int alRandomNum, string&asImageFile);
asTextCat - This is the category in the .lang file where to find the entry for the loadscreen text shown during a loadscreen.
asTextEntry - This is the entry that will contain the text message displayed during the loadscreen
int alRandomNum - This sets the amount of different text messages that can be displayed. Say if you set this as 3, for instance, you would need to add 3 different entries in your category for the loadscreen.
asImageFile - This is the picture to display, put the file name of the image here. no extension. Must be a .jpg, I haven't experimented otherwise.
Okay Example of the command. This can be put wherever your feel like it. The loadscreen will not change until you use this command again.
SetupLoadScreen("LoadingText", "Text_", 3, "TDG_Storage");
example of extra_english.lang
<CATEGORY Name="LoadingText">
<Entry Name="Text_01">Text</Entry>
<Entry Name="Text_02">More Text</Entry>
<Entry Name="Text_03">Even More Text</Entry>
<CATEGORY>
It will chose one of the 3 entries to display during the loadscreen. Your loadscreen image must be in a folder called "images" in your custom_story.
P.S if i made a mistake in the code, I'm sorry, it's late over here and a hurricane is incoming.