Every Box-Art image (stored in the BOXART subdirectory) has to be a 256 color Bitmap file with a size of 71x99 pixels. The colors of the box-art image are replaced with a best-matching color of the DOSMENU background image palette.
The file name of the box-art image bitmap must be the Entry-ID of the Menu-Entry (starting at zero).
If your Menu entries are these:
Commander Keen Police Quest 1 The Secret of Monkey IslandThen the Entry-IDs are:
0 = Commander Keen 1 = Police Quest 1 2 = The Secrent of Monkey IslandA box-art image filename for "Police Quest 1" must then be "1.bmp" inside the BOXART subdirectory.