Moons8800

hello i'm sorry for my english, i need help changing the index of my image ... because when i finish my image i save it in 15 colors ... then i use it on a utility ( bmp2Tiles) to convert my images to sega Master system format ... it worked before but today it does not want and I have an error message: ERROR image uses colours up to index 251-this must be no more than 15. There are 11 palette entries used.