Copy over the pre_menu.cfg file from /config to your own mod, then edit it. It should look like this to start off with:
You can leave the first line alone.
Next, you can change which banners appear at startup. You can edit/remove/add lines like these to use your own images:
<Section Image="your_image.jpg" Time="2" ShowFirstStartOnly="false"/>
Change name of your file, how long in seconds you want it and whether you only want it to show up the first time you start the game (mostly only used for the welcome and gamma screens). Usually the images are located in
graphics/pre_menu/.
After that you can change what is present for the first startup. The lines look like these:
<Section Image="welcome_bg01.jpg" ShowFirstStartOnly="true" Music="12_amb.ogg" MusicVolume="0.5" MusicFadeTime="10" >
<Text Pos="80 210 0" FrameSize="640 300" FontSize="18 18" TextCat="PreMenu" TextEntry="WelcomeMessage01" Color="1 1 1 1" Align="Left" />
</Section>
There are quite a few more settings in here. Again, use the name of your image, whether you want it to appear only first time (which you probably do), then comes music during this sequence. Edit volume and fade time.
As for the text, without having actually messed around with it, I assume the settings for the position is X and Y co-ord, though I'm unsure what the third number is. FrameSize is probably how big the text box is, so it knows where to cut the lines into paragraphs so they don't jump off the screen (bit unsure). FontSize is simple enough, X and Y size. Next is category and entry in your lang file. These are important to match, or else your game crashes. Have them point to your PreMenu category. Color is RGBA format (Red, Green, Blue, Alpha) and lastly you can have it allign to Left, Center/Middle or Right (if I remember correctly, more might be possible).