How you insert the sign depends on which editing software you're using. In Paintshop Pro, you select a box the size and place you want on the texture map. Then select and copy the new image in a separate window. After this, you use the "paste into selection" command on your original texture and save.
These instructions are dependent on your knowledge of opening tgpcx files and converting them back to their original format after editing. The F40 uses a newer graphics format that is harder to work with. Take a look at the second half of this tutorial by Marlyman. It's the best, most concise doc I've seen on the new graphics formats. It's free by the way.
http://www.railworks.marleyman.co.uk/st ... ucts_id=50