Simple : TDM uses a "copy" of the intro screen bmp, with added text "using TDM Modpack" ... So you use this screen on "yourmod" folder too !
The file is "Intro.bmp", there are 2 of'em, located in Pictures\Games\screens\800x600 and 1024x768 folders (each resolution has his screen).
To alleviate confusion you should edit it yourself (with a paint software) and replace the text with what you want
