Well, fair enough. Let me try and help you.
You want to re-skin the Big Boy right? Well, you can still do that. It's a bit long-winded, but here's how.....
1) Download the aforementioned dxtbmp (it's free off the web)
2) Next, download a program called 'Irfan View' (this is also free) You will need this if you have only got MS Paint as your paint program as you require any lettering to be fully transparent, and MSP doesn't do that.
3) Find the folder with the Big Boy in it and make a copy of it. Paste in the copy and rename it (eg BigBoy2) This is the folder you will use to repaint the loco. We make a copy so the original is unaffected by any modifications.
4) Go into dxtbmp and open up the new file. You will have to go through the various sub files to find you one you intend to re-skin, but normally it will have the words 'body_art' on it.
5) In dxtbmp, click on Image and click Send to Editor. This opens up the paint program you intend to use by default (you have to tell dxtbmp which one you intend to use by clicking on the paintbrushes icon on the far right of the screen.)
6) Now comes the modifying. We'll assume that you don't wish to re-skin the whole loco (this can be very time-consuming) but simply wish to add your own railroad name on the tender. First of all, we need to blank out the existing 'Union Pacific' lettering, so, click on the dashed rectangle icon on the left of MSP, drag a rectangular box over an area of the tender, right click, and click 'Copy' . Then click 'Paste'. The copied area appears in the top left hand corner of MS Paint. Drag it down and cover the 'Union Pacific' lettering. Next drag another rectangle over the same area, right click and click Copy To. Save it in My Pictures as BigBoy2tender.Minimise MSP
7) Open up Irfan View. Here is where we add the lettering. Drag a rectangle again in the middle and click Edit. Then click 'Insert Text selection' This will bring up a font toolbar where you can decide what colour, size and style you want your lettering to be. Type your railroad name in the blank box and click OK. You may have to experiment to get the lettering centred properly, but when you are finished click 'File' and 'Save'
8) Maximise MSP and right click. Then click 'Paste From', find BigBoy2tender in My Pictures and MSP will position the new lettering where the old one was. Then save the work.
9) Maximise dxtbmp. Click 'Image' and then click 'Reload After Edit' The new tender will be loaded up.
10) This is the tricky part. You have to get the correct Alpha channels in place or they could show up grey or white. Click 'File' and click 'Save As' Try the various formats. The ones that usually work are either Targa With Alpha or Extended Bitmap, but if it doesn't work first time, try the DXT3 one.
Thats it. Good luck and best wishes,
Padster