They date back to when the game was first released, and Internet bandwidth was much less than it is now. In order to save on file size, the images were black and white, and saved at a fairly low resolution (by today's standards).
I had been receiving requests to permit people to substitute their own images in the game, and this is now a possibility in Hellfire 3.
There are two ways of doing this.
The first way - best for handling small numbers of images - is to use the Image Manager built into Hellfire. This allows you to select images to use in place of the generic locomotive class pictures, and also to use an image for individual locomotives. The Image Manager is accessed from the Game Settings on the opening screen.
Select the Custom Images tab, and click Image Manager.
Select either Loco Types, or Locos from the left-hand list, then select the actual Loco Type or Loco from the middle list for which you use your own image. Then simply drag and drop the file from a Windows folder, or your Desktop, into the blank space on the right-hand side. The image should appear in the space. Repeat as necessary. When you have finished - click Save to save your image choices. If you need to, you can click Undo to reset the images back to the point you last saved them.
The second option is best when you want to install a large number of images. The images need to be named correctly, using just the loco number, so for example, if you want to use an image for 03162, your image must be called 03162.bmp/jpg/gif etc.
Using Windows Explorer, navigate to your My Documents folder and create a new sub folder called HellfireImages (if it doesn't already exist) and simply copy your images into there. This technique does involve a lot of file renaming if you are using a collection of files you have sourced yourself, or you can take advantage of Ian Banks-Jones hard work and download a complete set of named, colour adjusted and cropped images. Simply decompress and copy the images into HellfireImages to give yourself a complete set of loco images. A link to the latest version of the images can be found in the forum at http://www.railrover.co.uk/forum/index.php?topic=673.0. Owing to restrictions on copyright, I cannot distribute these images as part of the game, but they are allowed for your own personal use.
I hope you find this useful, and you enjoy Hellfire with your own images.