WINDOW WINSTART
{
TITLE "In die Begin, met Oom Karoolus en Lente";
SIZE 640,480;
MODE IMAGE;
PICTURE <start.bmp>,OPAQUE,0,0; <-- There´s a problem!!
PROGRESS RGB(0,0,255), 10, 10, 450, 620, 20;
}

I´ve tested it, have you saved the bitmap as 8-bit bitmap?
"Images must be in PCX or BMP 8-bit palettized format" (from the manual)