Do you have a screenshot of what's happening?
You might want to look at these functions in the code archives. The first one i wrote and it will mask/alpha off the colour you pass across. So, if you draw your sprite and mask off the rest of the image with a colour such as RGB (255,0,255), then call my function, passing across the mask colour, it will add in the alpha information, making the mask colour invisible. It will leave the rest of the image untouched.
http://www.blitzbasic.com/codearcs/codearcs.php?code=1013
This function by skidracer, basically smoothes the edges of the mask area, so the mask colour doesn't bleed through.
http://www.blitzbasic.com/codearcs/codearcs.php?code=1131
You might want to look at these functions in the code archives. The first one i wrote and it will mask/alpha off the colour you pass across. So, if you draw your sprite and mask off the rest of the image with a colour such as RGB (255,0,255), then call my function, passing across the mask colour, it will add in the alpha information, making the mask colour invisible. It will leave the rest of the image untouched.
http://www.blitzbasic.com/codearcs/codearcs.php?code=1013
This function by skidracer, basically smoothes the edges of the mask area, so the mask colour doesn't bleed through.
http://www.blitzbasic.com/codearcs/codearcs.php?code=1131